Crash gambling was one of the first game modes introduced to the CS:GO betting websites.
On Crash, you essentially bet on a multiplier that determines your risk and reward. Once the game starts, the multiplier will begin to go up but at some point, it will crash. Theoretically, it can keep going into x100000 but the likelihood of that happening is slim.
If the multiplier does not reach your specified multiplier before it crashes, you lose. If it does reach your multiplier, you win your stake * your multiplier.
Some sites like CSGOPolygon lets you cash out while the multiplier is ongoing which is quite a nice feature. The higher odds you can catch, the better. But be weary, it may crash at any time.