It's basically a skill that allows a catcher to either pull a pitch from being off the outside corner to make it look better and have a better chance of the ump calling it a strike.
The opposite can be true as well. Eck was talking about it on a recent game where an opposing catcher caught a ball well in the zone, but he pulled his glove out of the zone when catching it so it made the ump think the pitch was a ball.
Until robot umps are brought in, pitch framing and having a steady hand will be somewhat important. It's not the end all be all, but it has a little value.