在文献上看到“two principal suface moments”，请问是什么意思？

wql413

它的单位是nm的平方，
定义是：surface moments with respect to a pair of perpendicular axes (where the point of intersection is the center of the particle) in the direction of the moments min (M1) and max (M2)
请问大家这是什么意思。具体该怎么算？