OTA = "Over the Air" reception of the Digital Broadcasts of your local network channels via a TV Antenna (...just like in the old days....). The Vip622 has an input for the antenna, and a built-in High-Def tuner for the OTA digital transmissions so that you can watch either the Satellite HD Locals or the OTA HD Locals.
Strictly speaking, if Dish is providing your High-Def Locals, you don't have to have to use the OTA tuner, but there are benefits to using them in addition to the Satellite locals. For one thing, you can record three High-Def Network shows at once - e.g., NBC and ABC from the 2 Satellite Tuners, and CBS from the OTA tuner.
Also, in general, the OTA HD locals will have better picture quality than the Satellite HD locals....