qu'est-ce que means what? It is used to form a question, e.I.: Qu'est-ce que tu fais ? = what do you do ? Or What are you doing ? You could also ask: Que fais-tu ?
What is (in) fashion?
Qu'est-ce que c'est la rentrée? Usually it means the beginning of a new school year when children go back to school in September, but it can mean any other time during the school year when they come...
that means : what is it ?
Qu'est-ce que tu as fait? means what have you done?