When you say that something is "not appropriate", it means that it's bad. But the word "bad" means that something isn't good in any situation, while "inappropriate" means "bad" for a specific situation. You tell what situation you're talking about with "for ___". For example:
Did you see what she's wearing? That's really not appropriate for work.
You can also follow "appropriate" with "to (do)":
That's a good song, but it's not appropriate to play at a wedding.
The word "appropriate" is intellectual- and adult-sounding.