A metaphor is not to be taken literally but figuratively. It uses figures to represent another truth. It is similar to a parable.
For instance; I am the vine, ye are the branches: He that abideth in me, and I in him, the same bringeth forth much fruit: for without me ye can do nothing. - John 15:5 Here, Jesus is not literally saying that He is a grape vine and we are branches. He is using this figuratively. He is using a metaphor to help us to understand our relationship with Him, and how much we need Him.
Webster's definition: A figure of speech in which a term is transferred form the object it ordinarily designates to an object it may designate only by implicit comparison or analogy.