From Hard Drop - Tetris Wiki
The Ghost piece, or ghost for short, also called shadow or (in Arika games) Temporary Landing System (TLS), is a shaded representation of where the current tetromino or other piece will land if allowed to drop into the playfield. It is most helpful for lining up Hard and Soft Drops, especially when the stack is very low. It is generally colored fainter than the falling piece and the blocks in the playfield. As the player moves the falling piece, the ghost piece moves below it; when the piece falls far enough that it overlaps the ghost piece, the falling piece is always drawn in front.
Older games did not have a ghost piece, but all games that conform to the Tetris Guideline allow the player to use a ghost piece at all times, and Dr. Mario for Nintendo 64 has a ghost piece as well. The ghost piece reduces the number of misdrops, especially for beginners or for high-speed players who use hard drop, but some players who are migrating from games without a ghost piece have trouble adjusting to the ghost piece when they fail to distinguish it from active piece.