What does tu eres una fresa mean?

It means, "You're a strawberry." It also means, "You're a rasp (or drill)." To be a 'fresa,' has a pejorative connotation related to being snobbish, pretentious, and at the same time, contemporarily 'hip.' You may want to check out the link, below. It's in Spanish, but it gives several examples of "fresa-speak."