The altar is a place where witches cast spells, brew potions, do rituals… It is usually a small coffee table or a stool covered with an altar cloth. It’s important in which direction your altar is turned. It can be towards north, south, east, west or everywhere in between, depending on your tradition. At the altar, there are several different things that have to be there (that depends on your tradition). Every altar has to have candles. Then, there is cauldron (to brew potions), your book of shadows, athamé (ceremonial dagger), mortar and pestle, different symbols (pentacle…), and finally incense, to create a perfect atmosphere for magical activity.
P. S.: You can read more about tools in witchcraft on the page Tools.