That's what it means - junk. It's the bits of rockets, lost nuts and bolts and spanners that slipped out of the astronauts' grasp, and just float around the Earth. Thousands of bits of rubbish, too expensive and too much of a hassle to collect.
They're all tracked by NASA, so that when when they launch a new rocket it doesn't collide with the potentially damaging bits of spacejunk and ruin a mission (and all the money, time and work that went into it)