An API (Application Program Interface) allows other programs to be able to securely access your Realvolve database. What this means is that Lead Generation Website designers can setup their systems to AUTOMATICALLY add Leads (Buyers / Sellers) to the Realvolve database WITHOUT the need for Zapier.com. In addition to that, once the lead is in your system a workflow could be started from their side also. Another use would be for sending Showing Report information to a particular Property in Realvolve.
They do have to do some programming on their end, but many of them are VERY HAPPY to do this since it helps them make you very happy.
More Info: Realvolve API Docs