Overselling means offering more resources than possible to allocate. A common example of this is selling hosting with Unlimited diskspace and Unlimited bandwidth.

Almost all reseller's offering overselling start out the same. Clients have great uptime, servers are fast etc. As more time goes by the server becomes more and more filled from resellers using what they purchased and downtime gradually gets worse. People who purchase reseller plans with overselling enabled are going to use all of their disk space and bandwidth given enough time. The host has no control over future growth.

If you want to enable overselling, you need a VPS or dedicated server, but remember you can only fill the server with so many sites before the CPU and RAM become overloaded.