The theory is that you launch a telephone pole sized satellite into orbit, made primarily of some heavy, dense material like tungsten. When someone who annoys you masses their forces, you fire a kicker motor and send the pole down travelling to hit the ground at orbital velocities, like a directed asteroid strike. All the power of a nuclear bomb without all the messy fallout.

It's one Jerry Pournelle's pet concepts. Problems with correctly targeting something at terminal velocities, or for that matter why launching something that heavy into orbit is supposedly cheaper than an airstrike were handled by fairies not adequately explained.