From Hard Drop - Tetris Wiki
Tetrominoes fall from above one at a time. The player must sort them such that he or she forms continuous horizontal line clears, which will then clear, and the above tetrominoes will fall. Frequently, clearing multiple lines simultaneously will benifit the player. For example, clearing four such lines yield a tetris. As the game progresses, the tetrominoes fall faster, making things more difficult. Once a player tops out, the game ends.