What's Reiki

What's Reiki. The answer to that probably changes a bit (or a lot) with each Master.
The best one I could find is here , with the Firefly Society.
For me, I can start by saying that it's not a healing therapy. Sorry Bro.
It's a personal spiritual practice, and because of that it becomes a healing path and a healing therapy, among other things.
The healing, both on oneself and on others, comes however only as a result of a specific personal process.

Reiki is based on surrender and giving up. Not giving up something specific, or rather, not on giving up something external, but on giving up the ego-driven struggle for control.
Also, it's based on permission to receive light, and surrender to the fact that the light, or universe, or energy, or God, knows better. Which is something that rational people revolt against, although... Well to me it's completely irrational not to surrender. And I haven't found any smart argument (believe me, I looked, surrender was never my first option. Or second. Or third. I was dragged there, very much against my agenda. No one likes to feel hurt and little - but that's my point: surrender makes you infinite: beyond size there's only essence. That's what she said?) that can rationally articulate the opposite.

I think about all the random , completely uncontrollable sequences of events that brought me where I am. My birth, the beginning of everything in my perspective, was something I had no control over - with that kind of start, can I really expect any actual control? And if so, would I want it? I'm not talking about lack of responsibility or will, I'm talking about the fact that every decision I make is seemingly my own, meaning it's seemingly conscious, but it's actually heavily influenced by a series of subconscious psycho-emotional inputs that the ego wants me to believe I can control, but that as a matter of fact I can't even see.

It's what we'd call conditioning, but it's not just external, it's mostly internal as a reaction to the outer and inner workings of the world. That's the problem, we know what we know, we know what we don't know, but we have no clue that what we don't know exists can... well, exist. Let alone have a role in our lives.

We're thrust into this world, a finite shard in an infinite ocean, we feel our edges to get a sense of who we are and where we stand,  we feel them by rubbing against the outside, and it hurts, like snakes change skin by rubbing against sharp rocks.
More animal metaphors! We build an identity like bats send out waves into the darkness and get a mental picture of what's going on based on what bounces back. And because we're born crystallized in a little finite shape, we feel the need to get a rally clear picture of what that shape is, lest we end up swallowed by the infinite dark around us.
And so our so called identity becomes based on exclusion instead of inclusion, on recognizing ourselves as separated from something.
When I've made any decision in my life, or had any thought or reaction, everything was filtered by agendas, beliefs - Fears - that I built up, more and less consciously, based on this bat-like feedback geared toward, basically, survival. Not only physical, but also emotional and psychological. In a word, protection from the pain of death/disintegration/loss of self.

Which is fine, and a great first step. We need it, to get any sort of ground under our feet, however frail. But it's just that, a first step, it's an identification based on fear, a distortion of reality necessary to gather some seeming balance before deciding that the only intelligent thing to do with that balance is using it to gather momentum and jump in that infinite dark.

That's what we truly are, infinite, and until we decide to accept it, to embrace it by jumping in it, letting our will and perceived identity catch up with the will of the universe and our actual identity, we will always be detached, cut up, crystallized, pursuing something but also hiding and running away from something.
Maybe we'll feel strong and in control, but any strength that's based on exclusion and separation isn't real strength, it's just a very well disguised form of fear. Which is totally fine, it's just that, personally, I'm tired of being afraid, I'm tired of what I know I know and I'm tired of the reassuring self-feeding cycle I built for myself.

So, back to surrender: all the times that for whatever reasons  had the opportunity (read: was forced to) move past fear and be in touch with my self, whatever pain I was afraid of an whatever I felt  I always landed in a much, much better place. And learned who I am a little more.
With this information, I decide to trust the only mechanism that has actually worked, underneath all the little tricks and walls and adjustments I created, that I thought kept me alive and in balance but that in reality where hurting me and limiting me (and others around me because of my doing) as much as they were giving me a sense of safety.
I let those go, jumped and felt like I was quite literally dying, lost all balance and sense of myself, and then landed where I always was all along: safe, in the Infinite. In reality more than a jumping it's an opening of the eyes to realize we're already fully connected, already saved, already enlightened, already plugged in. That's enough synonyms.
We go through this cycle of "resurrection" again and again, like all the myths, religions, philosophies across times and spaces try to teach us. If we jump we move on, if not we stay stuck in the same stage of the cycle, until we're ready and willing to move on.
Humans are awesome and mighty, we have access to the Infinite in ourselves but we're not The Infinite, we can't possibly have that perspective, so let's trust in something that does, and by doing so we get to see more and more, and better and brighter, we get to move in that Infinite, become more aware of it and see and live like It does.

Surrender is rational because it's accurate, it reflects the actual order of reality. Like Spinoza would say, rationality is accuracy. Well he wouldn't say it like that. He would say it in Dutch.
The expectation that we have any full understanding or control in this sense is pretty inaccurate. And with inaccurate information, no stable, trustworthy systems can be built.

Supersummary: Reiki is a way to surrender and a way of actively building solid ground in one's life, asking for permission to receive Light and to share it with others. And if there's any extra, to deliver it to others, asking for their permission to do so.
In practical terms, in the setting of Reiki as a healing therapy, Reiki becomes re-activating&nurturing a connection from Light through Master to Client, by laying of the hands and energy channeling.

If you'd like to know more, read on.