What do you think this means?

Once in a while, all of a sudden in my head I see the face of someone I know. The weird thing is that I'm a VERY forgetful person and if i don't see a person often, I'll forget what they look like in an instant. In my minds eye, I CLEARLY see someones (usually an acquaintance) face, like sometimes it look like they're talking. It's so vivid and strange. What does it mean and what is it?