A short stack is a stack that's too small to cover the likely betting in a hand. A player who has such a stack is said to be short-stacked. This has advantages (e.g., that you cannot be pressured to fold) and disadvantages (e.g., that you cannot get maximum value from your winning hands). Asking whether or not this is a good thing over all is a good way to start an argument. The phrase "short stack" can also refer to the players at a table (especially in no-limit or pot-limit play, often in a tournament) who have the least money in front of them.