Answer: An attack where the attackers will interrupt a data transfer happening between parties and in which they will pretend to be the legitimate parties.
Explanation: For example think about two people writing letters to each other back and forth. However you, the attacker can intercept the letters and effectively change the message/contents of the letter going to the other person. This is probably not the best explanation, but simply put a man-in-the-middle attack is when an attacker interupts a transfer and pretends to be the legitimate source.
