x1 (li; tensor with units of kg×m2) is the moment of inertia/angular inertial-mass/rotational inertia of object x2 around axis/point of reference x3 in basis/coordinate system x4.
Physics is all about expressing the same thing in different ways in order to gain new insight about it. There are other ways to express a mathematical quantity which is equivalent to this one (much as gravitational charge and inertial mass are mathematically mutually equal but not conceptually identical).