Crash game
A multiplier climbs from 1x until it crashes. Cash out before it does and you win.
Every crash round starts at 1.00x and the multiplier goes up from there. Where it crashes is already set before the round starts. Cash out in time, yourself or with auto cash-out, and you get your bet times the multiplier. Wait too long and your bet is gone.
How the crash point is set
Everyone plays the same round, so provably fair casinos usually don't use your personal seed for crash. Instead, they build a chain of hashes ahead of time and publish the hash of the last one before the game starts. You can check each round against it, often mixed with a public value like a future block hash. Every site does it a bit differently, so go by their own docs.
Careful with the pace
Rounds are fast and everyone's playing together, so you can burn through a lot of them on one metro ride. The house edge usually comes from a small chance the round busts right at 1.00x, plus how payouts are calculated.
Real-life example
You bet 10 $ with auto cash-out at 2.00x. The round hits 2.00x? You get 20 $. It crashes at 1.73x? You lose the 10 $.