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 poetry is about noticing what the writer puts in, even if it's not exactly what the writer intended you to see. When it comes to symbols, what they mean to you is not always going to be the same as what they mean to the poet -- if indeed they weren't accidental, or subconscious. When we give you a poem, we give you the power to make your own meaning.
Looking over my own poems, I notice a couple of recurring symbols and I thought I'd share what they mean to me. Let me just reiterate: this is not to say that they should mean the same thing to you, although they might. If they do, hit me up -- we might be secret besties.
The dandelion drifts on the breeze, spreading its seeds wherever the wind goes. To me, this represents freedom of thought; the drifting fluff is not restrained by roots and need not stay close to its origins. It follows whatever current catches it.
The lizard is cold; it relies on the outside world to warm it and keep it active. If left alone in its home, the lizard would sink into torpor. It has a tough skin to protect itself, but it needs to be exposed to reach its potential.
I grew up in the bush (though not quite this remote) and I love the quiet and solitude, but the desert is more than that. It is harsh but always fair; those who are prepared and treat it with respect survive and thrive. You never really know the desert, you just accept it.