Silly describes something that can't be taken seriously. You might not mind telling silly jokes, but you don't want to be told a project you've worked hard at is silly.
Silly originally meant “happy,” which gives the sense of children's games or harmless jokes. Later, it came to mean “foolish”: silly ideas are looked down on because they're not very smart. Silly can also mean a state of foolishness brought on in a specific way. Your mother “worried herself silly” when you stayed out late, and promised to “slap you silly” when you got home.