"C'est quoi ?"
This is the most common and natural way to ask the question in informal French. It literally translates to "What is this?" but is used to ask for clarification or to ask "What is it?" about something you're unsure of.
Other options:
* "Qu'est-ce que c'est ?" - More formal, literally means "What is that?"
* "C'est quoi ça ?" - More casual, literally means "What is this?"
* "C'est quoi ton truc ?" - "What's your thing?" (used to ask what someone is up to)
* "Qu'est-ce que tu fais ?" - "What are you doing?"
The choice of phrase depends on the context and your relationship with the person you're speaking to.