A Retrieval Cue is a prompt that help us remember. When we make a new memory, we include certain information about the situation that act as triggers to access the memory.
For example, when someone is introduced to us at a party, we don't only store the name and appearance of the new acquaintance in our memory. We also include external cues about the situtation like what kind of party it was, who made the introduction, what cocktails were served, or what music was playing. We also include internal cues like what mood you were in at the time, or what you thought of the person being introduced. When we try to recall the person, having one or more of these cues present will help us remember better. So when you meet the person again, it would be easier to remember them if you bumped into them at another party, or you saw them with the same person who introduced you, or you were in the same mood as when you first met them.