An S-Block (also known as a reverse squiggly) is a tetromino that looks like an S. In most games it can only clear up to two lines at once. However, in some games that use SRS, it can clear up to three lines at once.
It is also one of two antagonists in the game, the other being the z-tetrimino. This is because it is inconvenient, especially when you are set up for a Tetris and you get this tetrimino.