Each round of a crash game begins with a betting window — usually a few seconds — where you place your bet. The multiplier then starts rising from 1.00×. It climbs gradually, then suddenly crashes to zero at any moment. There is no fixed time limit for when the crash happens — a round can end at 1.05× or go past 100×.
You must press the cash out button before the crash. When you do, your bet is multiplied by the current multiplier and added to your balance. If you cash out after the crash, the bet is lost entirely. This single rule is what makes every round tense.
Every round is completely independent — the multiplier from a previous round has no effect on the next one. Three consecutive low multipliers does not mean the next one will be high. Misunderstanding this is one of the main reasons players run into losses.