A pestle is a heavy, blunt tool used to grind things up, such as spices or herbs. If you're grinding spices, you put them in a container called a mortar and use the pestle to smash them up until they're finely ground.
The mortar and pestle was originally used by pharmacists to grind substances for medical use. Because they are sold together as sets and are always used together, many people aren't sure which is the mortar and which is the pestle. The origin of pestle is the Latin word pistillum, which means "pounder." This can help you remember that the pestle is the pounder. Also, pistillum looks like pistol, which is also held in the hand.