It's a simple concept: the repetition of a letter or sound. Easy enough.
The wobbly walrus waddled toward the wall.
Alliteration is great for poetry. A wonderful example is from my favorite poet and writer, Edgar Allen Poe.
"Once upon a midnight dreary while I pondered weak and weary"
(notice how even though Once starts with an "o" it makes a "w" sound, so it's included as part of the alliteration)
I won't highlight the entire poem for you, as there are many examples of alliteration throughout (and you might begin to feel like you're in a college poetry class). But that one example shows us the fluidity it brings to the poem.
So why the heck is it important in novel writing? And is it necessary?
Well, for one, it could add a layer and flavor to your writing that might not otherwise be there. For another, alliteration has a natural tendency to feel familiar, and brings your reader a relate-able language (so long as it's not overdone). But no, it's not necessary.
I do prefer to use alliteration in my writing. And many times it happens without my thinking about it. Guess that's the inner poet in me shining through. :)
What about you? Do you use alliteration?