What happens when you backup your iphone?

I just went on itunes and backed up my iphone, and it loaded, and finished. Now what? I'm new to this so don't be mean please

Nothing really. It makes a copy of your information that's stored in either your computer or iCloud. In case something happens to your phone in which you have to get the whole thing reformatted (basically delete everything), then you can restore your information in iTunes through the backup. Hope this helped.

It saves all your stuff so if you restore it, you get everything back

Well, then you have a copy for all your personal data on the iPhone and can restore them if you accidentally deleted them or lost them somehow later.