An olive pitter is a tool to remove the pit (aka stone) from an olive.
You put an olive in the pitter. One side of the pitter has a small cup with a large hole in it. The other side has a punch in it. You press down and the punch passes through the olive and presses the pit out through the hole.
An olive pitter is the only way to pit olives at home without chopping or mangling them.
This might seem like just another gadget to clutter your drawers, but if you frequently pit olives to make a dish such as tapenade, you may see its usefulness.
Olive pitters can also be used to pit cherries.