An easy way to understand metaphor is to view a metaphor as a simile without the word "like". A simile compares two things in a clear fashion. For example: "he is like a beast". Another example of a simile would be to say that the man is so fierce that he is like a tiger. A metaphor assumes that the relationship will be understood without the interlinking word "like".
Metaphor works on many levels in poetry. The best way to show how a metaphor function is to study the use of sustained metaphor. Sustained metaphor refers to a metaphor that consistently runs through the entire poem and is therefore easily identifiable. Metaphors that are sustained also provide a depth and inner complexity to the poem