Don't try to pretend to be better than you really are

Hmm...it depends on the context of it in the sentence. In its purest form, I would suggest that it means "Don't try to show off" with the implication of 'if you do it like that, you'll end up looking stupid despite trying not to'. However, I think I've heard it used as a "don't be funny with me" or "don't be 'smart' with me" (both idiomatic expressions that I can't really think how to explain, but they usually precede a fight).