A metaphor is an imaginative way of describing something by referring to something else which is the same in a particular way. For example, if you want to say that someone is very shy and frightened of things, you might say that they are a mouse.

If one thing is a metaphor for another, it is intended or regarded as a symbol of it.

If you mix your metaphors, you use two conflicting metaphors. People do this accidentally, or sometimes deliberately as a joke.