I started reselling because of the nature of my work - I needed a system whereby I could offer clients a one-stop-shop in terms of web design, hosting, and email services, and reselling was the answer.
At its simplest it's very straightforward - you offer people domain name registration and hosting packages, and hope that they'll sign up. Unfortunately it doesn't stay that simple for long - you'll be involved in servicing the clients around the clock, 365 days of the year, and where websites and email are concerned people don't like to be kept waiting for very long - they need answers, and they need them quickly.
It helps to know a bit about server configurations and domain name administration, and you definitely need a good service provider - modern data centre facilities with 24/7/365 monitoring and redundancy. The very last thing you want is for a server to go down and stay down on a Friday evening before a public holiday.
I know of nobody who makes serious money as a reseller - I certainly don't, but then I didn't go into it with that in mind. You can certainly make some money if you work at it, but it is hard work - the competition is fierce.