When there is a back in during a game of poker, a player must have entered a pot after checking before calling another player's open. This is usually done on the first round of betting and is more common in poker games such as Jackpot. Once an entry is made into the game but there are no openers, then a back in has been made. There are some moves that an opponent can make to counter these moves but it depends on which of the poker game is being played.
In a poker game, the term back in occurs when a player has a strong hand and is well positioned for a strong move, s/he waits for the next player to bet before calling for a raise so as to hold on to the advantage offered by the lead. This move is only made possible in some types of poker games where the players are allowed to raise and check as well. For some games, a re-raise is equally allowed and another player can call before or after the raise is done.