All-in is a high-risk move in poker. It involves the Player betting all his chips. He moves his chips in front of him to the center of the table and stands up, so that all the other players at the table know that a player has gone all-in.
This simple move can turn the entire game upside down in a very short time. Anyone who goes all-in risks losing all their chips at the end of the round and may have to leave the table (e.g. in Tournament play).
There are only a few situations in the game when it makes tactical sense to go all-in. This is, of course, especially the case when the player is absolutely sure that he or she has the best hand and expects at least one of his or her opponents to call. However, it can also happen that a player has to go all-in to stay in the game. This is the case, for example, when the player has very few chips left and another player has raised the Bet so much that you can no longer pay the raise.
If all opponents have folded and the Pot is relatively empty, it makes little sense to go all-in, even with a very good starting hand. Then there is a high Risk that the opponents would be intimidated by an all-in move and would fold. Then, despite a good starting hand, the last player is left with only a comparatively low win.
Depending on the game situation, it may also make sense to go all-in in order to bluff. However, this is an extremely risky move.