It's a question used to ask someone to repeat what they just said because you didn't hear or understand it.
Here's how it breaks down:
* Que: This is the Spanish word for "what".
* Dijiste: This is the second-person singular past tense of the verb "decir" (to say).
So, "que dijiste" literally translates to "what you said" but is used as a question meaning "What did you say?"