“Don't just sit there! Do something!”
You are busy cleaning up your house because there are guests coming over for an hour. Your children aren't helping and are just watching TV. You complain this to them.
Don't just sit there! Do something!
This means to sit in one place and not do anything. You use this phrase when criticizing someone, when you think they should be doing something. You can also say "just stand there" in a similar way:
Don't just stand there! Help me!