Buying/Selling Traffic . . . how does it work?

First of all i wanna start off by saying im not interested in either buying or selling traffic

this may be a stupid question but you dont learn anything if you dont ask what i want to know is how it works? i mean the people selling the traffic where do they get the traffic from? do they simply buy it from somewhere else and sell it to you for more? or maybe use those autohits sites and just do the 'leg work' for you? or is it just traffic they already have visiting one of thier sites and they re-deirect it? i just dont get it

please someone explain

Thanks