"What don't you like?"
Translation:Was gefällt dir nicht?
The link provided by az_p is a very good explanation. You should read that in its entirety, but to summarize:
- Mögen is a modal verb and the literal translation of the English "to like".
- gefallen is an intransitive verb meaning "to please" or "to appeal to"
And although you didn't ask about it, you should still know about gern, which is an adverb that describes an activity. It's meaning is something akin to "gladly". I'm not sure how one would use it here (perhaps "Was hat dich gern nicht"), but to provide at least one example: "Ich lese gern" means "I like to read."
That's the way gefallen works (Wiktionary entry here). Think of the verb as "pleases". So, an alternate version of the English would be "What is not pleasing to you?":
- "What" is the subject: Nominativ: "Was"
- "is pleasing" is the verb phrase: "gefällt"
- "not" negates the verb: nicht (NB: auf Deutsch it moves to the end of the sentence)
- "to you" is the indirect object: Dativ: "dir"