The wikipedia definition is pretty scary for Moment of Inertia

http://en.wikipedia.org/wiki/Moment_of_inertia

but it is basically the resistance to rotational acceleration about a given axis. Within SolidWorks, the axis is defined by the subscripts. The difference between the L and the I numbers are that the axis is different. The I is an axis through the model's coordinate system. The L's are an axis through the center of mass.