What is a metaphor?

A metaphor is a way of explaining something in terms of something else, without using the words "like" or "as." It is an implied comparison. I think the best way to help you understand this is with some examples. 

In explaining that I have a difficult task to do, I might say, "It's going to be long haul." My task, which might be grading a large pile of papers, has nothing to do with hauling anything anywhere, but it gives the listener an image of someone having to do a lot of work by hauling something very heavy over a long distance. That is a metaphor. 

I might write that someone who had an unpleasant surprise was shell-shocked—in other words, shocked by the form of bomb called a shell, which one could be attacked with in a war. Everyone reading this understands I am making a comparison with a person who is attacked, but that the person I am speaking of has not been attacked, and is instead just unpleasantly surprised.  

In each instance, the reader or listener must be able to understand what attributes of the metaphor are meant as the comparison. Otherwise, the metaphor cannot be understood properly. If I say someone is a rock, I probably mean the person is strong and someone I can depend upon.  A misinterpretation of that might be that this is someone who is very cold, unemotional, or who has rough edges.  These kinds of misunderstandings can happen for the listener or reader who does not have a good familiarity with the language being used. Young children also frequently misunderstand metaphors because they take them literally. When I was a child, I remember my father saying someone he knew was fired, which probably did start out as a metaphor. I was terribly upset because I had the image in my mind of a man on fire. This does show us, though, how strong an image a metaphor can provide!