Reseller hosting is when you buy lots of space and bandwidth along with domain names from a web host.
And then selling chunks of that stuff to other people while charging them more then the web host charged you, in order to recover what you spent on the web host and making a little profit for yourself
The good thing about reselling is that you don’t have too many headaches, the web host managers the servers etc. for you, now the bad thing about it is that the support takes a bit longer to be sorted out, as first you have to forward a customer query to the web host, then the web host resolves the problem and then the reseller gets back to the customer, so the whole process takes a bit of time.
A good web host with 24/7 is really necessary in these cases to speed up support.