Definition all in
When a player is all in, it means that he has put all his chips in play. In French, we can say that he has gone all in or that he is at “tapis”. We can be all in for several reasons, but let's name two: to maximize our chances of bluffing and to maximize the winnings of our hand. If we think your opponent has an average, but not excellent hand and we want to make him fold, being all in may convince him to fold. And if we feel that he has a very strong hand and we have an nuts, we'll want to be all in to maximize our winnings.
Psychological aspect of all in
When a player is all in and is in a casino or in any live game, he will have to announce loud and clear that he is all in. Knowing that one is all in can be interpreted in different ways by our opponents. If it is a shy player who is all in, he will usually have the best possible hand. If he is an aggressive player, he may try to intimidate us with his "all in" to encourage us to fold.