Overselling means offering more resources than possible to allocate on a server.
Hosts that offer overselling on their Reseller Hosting Plans start out the same. Clients have great uptime, server performance is outstanding, but as time goes by the server becomes more and more overloaded as resellers add more and more hosting accounts, and as a result server performance gradually gets worse. Resellers who purchase reseller plans with overselling enabled are typically looking to offer unlimited diskspace and unlimited bandwidth, which in reality doesn't exist.
If you want to enable overselling, you will need to get your own dedicated server, but understand, you can only fill a server with so many sites before you deplete all the server resources.