The base of something is usually the foundation, starting point, or main ingredient of something. A soup base is the flavoring or broth you use to get your soup started.
If you're a soldier, you might live on a base ("place where you're stationed"). When your unit plays softball, you must touch each base before scoring. The bottom of pentagon-shaped home plate is its base ("bottom"). Yelling at or pushing the umpire is considered base ("mean-spirited") behavior. At end of the season, your team might get a trophy, which sits on a base ("pedestal"), or a plaque made of brass, an alloy of zinc, which is base metal (it corrodes easily).