How do you say what are you doing on the weekend in french?

Que fais-tu is a French equivalent of 'What are you doing'. The interrogative 'que' means 'what'. The verb 'fais' means '[you] are doing or making, do do or make, do or make'. The subject pronoun 'tu' means 'you'.

That's what a speaker may ask of someone from the close circle of family, friends, and peers. Otherwise, it may be 'Que faites-vous' or 'Qu'est-ce que vous faites'. Both phrases use the more formal 'you are doing' ['vousfaites'] form. They respectively are pronounced 'kuh feht voo' and 'keh-skuh voo feht'.
qu'est-ce que tu fais ? is the French translation for 'what are you doing'.