Definition of ss
SS stands for "shortstack". There are two main reasons for being shortstacked. When you are shortstacked in a tournament, it's because the blinds have increased a lot and you haven't won enough pots to keep yourself at 100bb (100bb is what we call a classic stack, it's a reference). When you are shortstacked in a tournament (say between 2 and 30 big blinds), it is not by choice, but by necessity. When you are shortstacked in cash games, it is because you prefer a game with fewer chips. Often, shortstacks will have 20 bb and will only play 2 streets most of the time, preflop and flop. These are called pro shortstacks.
The shortstack pros
A shortstack pro is a player who plays cash games with 20 bb's consistently. When he doubles up, he leaves the table to join a new one with his 20 bb. This strategy avoids having to play multiple streets. It's simpler poker. The majority of the time, the shortstack player pushes or folds. These players also take advantage of the time that many players will misfit against shortstack players.