Apple has never been completely clear on what it does, but as the name suggests, it makes the network more robust in an environment where there is a lot of interference. My understanding is that it should be used if you're experiencing wireless issues due to a nearby microwave oven, which operates on the same frequency. (2.4 GHz cordless phones also operate on that frequency, but I think I remember reading that the interference robustness is specifically designed to deal with microwave interference...not sure whether that's true or not.)
If you're not having any problems with it turned off, I wouldn't turn it on. It decreases your range and/or speed, but it makes sure the data gets through.
And I believe it must be turned on both your computer and your base station in order for it to work. One isn't enough.