Let's say "reality" is the part of existence you can experience. Sometimes that's direct: you see, feel, hear something. Lots of times it's not; something just happens, and if you stumble across it or perceive it somehow, it becomes real for you. You've experienced reality.
Your perception is different from someone else's. As an older white guy, I move through reality with a different set of perceptions than might an attractive young Asian woman, or an aging billionaire. We might all see the same shaker of salt on the table, glance out the window at the same sea (the Aegean for me, as it happens), or run into the same weird coincidence — but we experience those things differently, and they spin off different thoughts, memories, and potentials for each of us.
As I get older, it seems like less and less of "reality" is really objective for all of us.
I also find it's pretty bendable. Decide you're going to see more blue cars, and you do. At first maybe they were always there and you're only now noticing them... but the intention itself seems to have effects that go beyond noticing, effects on the way reality unfurls in front of us. I have my assumptions about how things usually go. It's my experience that those assumptions can be wildly exceeded.
For the record, it seems obvious to me that the material of reality is bendable, fungible. I can tweak it; sometimes by changing my perception, sometimes with more focus, sometimes with an act of will or something more intense and drawn out. By "tweak" I mean: bring about an occurrence that wasn't going to happen before I tweaked it.
Austin Osman Spare had a word for the substrate this happens in: Kia, the life-force or "I" behind all the little selves and beliefs we wear. Spare's method was less about commanding Kia and more about getting out of its way, stripping off the beliefs, habits, and assumptions that clutter perception so that will could move through unobstructed. I find that useful. It's less "I'm imposing my will on reality" and more "I am clearing the way so my will can land."
So magick, for me, is the intentional bending or tweaking of reality.
Aleister Crowley (a Western ceremonial magick practitioner of some notoriety) defined magick as "bringing about change in accordance with will." I accept this, but I prefer the more glib version I picked up from Tommie Kelly, who can't remember who he heard it from either: getting lucky on purpose.
For a lot of reasons, practicing magick — getting lucky on purpose — usually involves more, and ironically also less, than simply deciding what I want. There are things I do that increase the likelihood of my will being encoded and translated into something that manifests in my reality. Spells, sigils, various flavors of ritual, connecting spirits and other entities... all of it comes into focus here, in the space between wanting and getting.
As I develop my practice, discovering on my own and also standing on the shoulders of giants, following those who have come before me, I get more experienced and more learned. I become more able to effect change in accordance with my will. More able to get lucky on purpose.
I continue to be amazed at what is possible, and where the boundaries are, or aren't.
And as I go, I still see the same salt shaker, the same Aegean, the same coincidence that everyone else does. But I've come to understand them in a different light, through a different frame of reference — one where nothing about that salt shaker or that sea is quite as fixed as it looks.