Redundancy receivers have the ability to add a second receiver via the SBUS IN port. In the event that the main receiver has a poor radio connection, it will automatically switch control over to the second receiver if it has a better connection. Some may think of this as a satellite receiver, typically this will be an X series receiver.
A very nice option with the FrSky redundancy system is the ability to create a dual frequency system for greatly improved safety. To do this, just add an R9 module to the back of your FrSky transmitter and then plug an R9 receiver into the SBUS IN port. Now you have a system that uses 2.4ghz and 900mhz.