Double up is common poker slang meaning to "double the size of your chipstack". This is usually used in big bet poker (i.e. no-limit or pot-limit) to describe the effect of a player pushing all-in and another player calling. The all-in player is hoping to double up "through" the non-all-in player. (Note that the player who is not all-in cannot double up - they can only win the amount that the all-in player had available to bet).