What is "Push"?

Ok so I know that this may seem silly but I have to ask. What exactly is “push”? I have an iPhone 3g and on it in my settings I have the option to turn push on or turn it off. I dont know even what push is, what it does or how it works, I read a few things online that would explain it to you but all those answers only explain it as if the reader already knows something about what it is. Sadly I need a beginners explanation. Please someone explain this to me from the very beginning. Preferably in laimens terms. Thank you.