"Qu'est-ce que tu as fait ?"
Here's a breakdown:
* Qu'est-ce que - This is a common way to ask "what" in French.
* tu - "you" (informal)
* as - the past participle of "avoir" (to have), used in the passé composé tense (the most common past tense in French)
* fait - the past participle of "faire" (to do)
Other ways to ask "What did you do?" in French:
* Que as-tu fait ? (more formal)
* Tu as fait quoi ? (informal)
* Quoi tu as fait ? (very informal)
The choice of which phrase to use depends on the context and the level of formality you want to achieve.