A player who goes all-in pushes every remaining chip into the pot. It works like a raise, except that the size is set by the player’s stack rather than chosen freely.
Stack sizes decide what happens next. If you move all-in for 50,000 chips and an opponent with only 20,000 calls, just 20,000 of your chips are at risk against that player and the rest comes back to you. A caller with a bigger stack only needs to match your 50,000. When several players are involved and stacks differ, the extra chips go into one or more side pots that the all-in player cannot win.
Once all-in, a player makes no further betting decisions in that hand and simply waits for the showdown. Under table stakes rules, nobody can be forced out of a hand by a bet larger than the chips they have on the table.