You have to put a lot of energy into a laser. You have to watch it, add energy at the right time, in the right way, and hope. Lots of hope. And a lot of stochastic variance falling into probable event horizons. (for most people, this is luck).
A laser needs a medium. It needs a place to happen. The first lasers used ruby. Red rock, semiprecious, fully lovely. Red from the chromium in it, which gives us lots of good atoms to do the dance.
Those atoms get charged. We don't really care how, but something excites them. A lot. They get all this energy, but then don't know what to do with it. Most of the time, they just fizzle, shedding the energy and emitting a photon in a random direction. We don't always get it right the first time.
A laser needs resonance. It needs some way to collect some of those photons, fizzling off in all directions. Think of a mirror on two sides of the room, that's fine. Sometimes those fizzles will go in the right direction, and reflect off a mirror, and come back through. When you get enough of them, bouncing from mirror to mirror, something happens. Something completely unexpected, something more than the medium and the energy and the resonance.
Our poor little atom, he fizzled... but now, a new energy cycle comes through, and he's all charged up again. It'll be better this time. This time, things are going to happen! He's waiting, waiting. He's getting lonely. He starts to feel like he will just fizzle. It was just a misunderstanding, he shouldn't have gotten his hopes up.
But one of those bouncing photons, a little piece of light, comes just close enough... and something magical happens. It's just right, the one he was waiting for. AND HE GLOWS.
The atom will emit a photon JUST LIKE THE ONE THAT JUST PASSED. In the same direction, contributing to the growing avalanche of correctly-bouncing light... making the laser stronger, brighter. Making it a better place, a place where more of these magical encounters can happen.
There's a lot of things they do to try and make this process easier, but it's beyond the scope of this little box. But all they really come down to is keeping that little atom excited for just long enough for something special to happen.
For it to find the one that'll help light up the night.