I am no expert but I think that wild symbols work in exactly the same way as the wild card in poker. Of course I could be wrong. We'll hear what those in the know have to say.
Wild symbols work by magically transforming themselves into whatever symbol you need to create a winning combination in the slot game that you're playing.
I think that's a simple enough explanation. Also, to help you understand, wild symbols are there to help to generate wins that otherwise wouldn't have been there.