MOI stands for moment of inertia.
http://en.wikipedia.org/wiki/Moment_of_inertia
In its most dangerous form, it was also part of "inertia-coupling" and is responsible for the destruction of many high-speed aircraft most notably the X-2. It was clearly a problem on the X-3 and F-101 and all "long" aircraft, which was most of the century series.
http://en.wikipedia.org/wiki/Inertia_coupling
AirWrench will calculate these for you based on your current entries, but only applies them to the FM if you choose to do so. You can save the FM with the old MOIs but with new data to get a better performing, if not completely accurate FM. It helps smooth out the limitiations in our non F-22 computer software.
In fact, it might big a big part of the dead barrel roll bug in our enemy ai.