It is a phrase used to inquire about something with which something else is done or used. It is a combination of the preposition "con" (with) and the interrogative pronoun "qué" (what).
Examples:
* ¿Con qué escribes? (What do you write with?)
* ¿Con qué te vas a vestir? (What are you going to dress with?)
* No sé con qué limpiar esta mancha. (I don't know what to clean this stain with.)
Note:
* "Con qué" can also be used as an interjection expressing surprise or disbelief. For example, "Con qué te vas a casar!" (What are you going to marry with!)
Hope this helps!