Okay, so I know that the index if the inertia tensor entries stand for the moment of inertia about the second index when its rotating around the first index. For instance, the top middle entry is the moment of inertia of y when the thing is rotated around the x axis. There's a square, and the origin of a coord system is placed in the bottom left corner. If you do out the inertia tensor, I12 is -1/4ma^2 but I13 is zero. I get why this turns out this way mathematically, but physically, why isn't I12 zero too? Its not really rotating about the y axis. What do the products of inertia really mean?