Play Behind
Play behind means that a player has extra chips or cash that is not in play, but available to use during the game. It is allowed in some cash games.
The simple explanation
If you have more money than you put on the table, it is behind. In some games, you can use it when you need it.
A closer look
The rule is that money behind can be played only if it was announced and allowed. Some rooms use table stakes, so only chips on the table count. Check the rules before you rely on money behind. In big home games, play behind is sometimes allowed by agreement.
A practical example
A player has $300 on the table and $500 behind, which he can use if the game allows it.
Full explanation
Play behind means that a player has extra chips or cash that is not in play, but available to use during the game. It is allowed in some cash games.
Play behind is extra money that is not on the table.
It is allowed in some games.
Table stakes rules do not allow it.
Check the rules.