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 the main antagonist in the game, even worse than Z. This is because it is inconvenient, especially when you are set up for a Tetris and you get this tetromino.