Just like regular ol' inertia, but used to describe things spinning rather than moving in straight lines. Analogous to mass in linear kinematics equations:

F = ma becomes <tau>=I<alpha>, and so on

Rotational intertia can be computed for a point mass spinning about an axis by I=mr², where r is the radius of rotation. For all other objects, rotational intertia can be found by integrating across the entire body as a collection of point masses.