Rapid is an adjective, though often adverbial, while repede is an adverb.

So to say about someone that he is quick, you'd have to use rapid:

• CORRECT: Tu ești rapid.
• WRONG: Tu ești repede.

To describe an action as quick, you can use either.

• CORRECT: El mănâncă repede. (preferred)
• CORRECT: El mănâncă rapid.