When something is not good, it's bad. A bad dog goes to the bathroom in the house, and bad milk has gone sour.
Bad has slightly different meanings depending on what it's describing, but the overall idea is the same: not good. If you get a bad connection on the telephone, there's something wrong with the line or phone. If someone gives you a bad check, there's no money to back up the check. If you do a bad job on a test, you got a C if you're lucky. In movies, a villain is a bad guy. Also, when someone does something wrong, they might say "My bad!"