A player that is currently into a game but does not have the best hand or even a good one is said to be behind. It is common to hear a player telling another one "before the previous deck of cards were dealt out, you weren't having the best hand." As at the time that all hands are evaluated, the person who is somehow limited due to the type of cards s/he is holding can be called a player who is behind. You may have heard some poker players say things like "She was very sure that her pair of Jacks was behind her opponent's Kings, but since she had other options as outs, she simply kept on playing."
It could be used to describe money that is in play but that may not be seen as chips in front of the player. A player may reveal for example that "S/he is $150 behind and at the same time handing over some money to the card dealer. This means that s/he wants those chips to still continue in the game from the time they are brought to him or her. Once a player gets below in hand, s/he is behind. That could be before a raise comes on board to the rescue probably after so many outs.