Things that are indecent are obscene, indelicate, or offensive in some way. If you run around the mall naked, you’ll get in trouble for indecent exposure!
Indecent describes things that are not decent — the prefix -in often means “not,” like how independent means “not dependent.” Decent things are morally sound, so indecent things are offensive. Swearing is considered indecent. Indecent often means rude. A joke might go over well with your friends but is considered indecent when told to a teacher. Not everyone will agree on what’s indecent, but we all agree that you should keep your clothes on at the mall.