Something that is used to represent something else in order to imply certain traits to an unrelated object/topic etc .... for example. "time is a thief".... you dont honestly think that the clock goes into people's houses and steals things... but the idea that is attached to thief's is then put to time. its a negative word, takes valuable moments like a thief takes valuable possessions.
Do you know what a "smilie" is? Any comparison that uses "like" or "as" - remember? A metaphor is a lot like a smilie - but doesn't use "like" and "as" - but describes a real thing with something more abstract - the idea is to get is that the relationship is the same or nearly the same - metaphors can help clarify some comparative relationship to your reader.
Rattling through the halls of my mind is the kernel of an idea: if I plant the idea, let it grow under the light of reason, it will bear fruit as a concept, born full as Venus Anadyomene, ready to affect her good pleasure on all who espy her. Good metaphors liken a abstract to a concrete and are useful in giving people an understanding of one or more aspects of the concept. in the example above, your task is to tell me how many metaphors I used.