Definition of SH
SH stands for "shorthanded". In French, shorthanded could be translated by "à court d'hommes". In other words, there are players missing. If in a field hockey game your team takes a penalty and one player is on penalty for 2 minutes, you are said to be playing "shorthanded". In poker, shorthanded tables are tables with 6 players. Regular poker tables are the 9 and 10 player tables found in most casinos. The purpose of playing shorthanded is to have tables with more action. The action is said to be faster and more aggressive in shorthanded.
SH in tournaments
When you are in a tournament, towards the end of the tournament, there will be times when you are shorthanded. Imagine a tournament with 16 players left. The tournament has tables of 10 players. There are 2 tables of 8 players left in the tournament. At this point, you will be shorthanded. Take advantage of this moment to steal a lot of blinds. Usually, tournament players are used to 10-player tables, so when they are shorthanded, they may be too tight. That's why even if you're a 9 or 10 player table player, you still need to work on your shorthanded game.