A mortar is a bowl-shaped container that you can grind things in using a pestle. You probably buy your spices pre-ground at the store, but if you wanted to be old-school, you could grind them yourself with a mortar and pestle.
Mortar is an old word — it comes from Old English, and using a mortar to smash things up is old way of cooking. It's also the builder's paste used between bricks. If you've heard the phrase "brick and mortar," that's a style of building, as well as a general phrase for something with a real building (as opposed to an internet business). And it's also a gun that fires high bullets that land short-range.