During calling of betting, an all-in occurs when a player runs out of chips to play. In a hand, a poker player might not be able to dip into his purse to source for more funds during a table stakes game. When this happens and a player has no stock of chips remaining, then a side pot will be made but the player will have no interest in it but s/he could still be able to win that pot in which he had some chips. If a poker player places their last chips inside a pot, it is said of the player that s/he is all-in.
When a player becomes all-in, that person is not entitled to win any bet placed above their last bet which is the side pot. This is even more common in a game like table stakes but the player may be entitled to take the main pot so can't be coerced from that hand. To put it into a sample statement, one will say something like this: "Sue made quads on a full house, however on the next bet she was all-in" or "I put him all-in after raising my small bet."