These are programs that pay you for multiple transactions from the same customers. For example I'm a domain name and web hosting reseller (that's why I push websitepalace.com) so I earn a commission for every customer that signs up for hosting and domains. So as each customer renews their hosting every month, year, etc. I continue to earn a commission on each transaction.
I usually try to find residual programs that fit into my niche. Unfortunately there aren't a lot of good ones out there. You can go to
www.lifetimecommissions.com to research more.