Intereference Robustness is a feature built in to Airport cards to help drown out interference from other wave-emitting devices in similar frequencies, such as microwaves and 2.4 Ghz cordless phones. From what I can gather, turning on interference robustness increases the power but reduces the transmission range, thus you need to be within a good distance of your Airport base station for this to work.
Apple doesn't give much information about this. I had to do a Google search a while back when I had the same question. So basically, don't use it unless your computer and base station are right next to the microwave!