What's basic is what's essential, at the root or base of things. If you've got a basic understanding of differential equations, you can handle simple problems but might get tripped up by more difficult ones.
You can also speak of the basics. If your kitchen is outfitted with just the basics, you've got some bowls and pots and pans, but you probably don't have an apple corer or a candy thermometer. If you're hitchhiking across the Gobi Desert, you'll want to have the basics of conversational Mongolian down. You've probably heard basic in chemistry class, too: it's the opposite of acidic. A basic solution has a pH greater than 7 and turns litmus paper blue.