A symbol is another one of those lovely shortcuts to meaning that poets are so fond of, a word or image that represents an idea. If you don't "get" all of the symbols in a poem, you should still be able to read and appreciate the poem -- but picking up on these little abstractions can really enrich your interactions with a text. Of course, symbolism isn't unique to poetry, but permeates every part of our lives... we just don't necessarily notice it. Being a good reader of p