As Phil mentioned in his answer, neither the novel nor script are much help, referring to it only as debris and/or junk:
The hatch on the underbelly of the Avenger yawned open. And as the
Imperial galactic cruiser zoomed into hyperspace, it spewed out its
own belt of artificial asteroids - garbage and sections of irreparable
machinery that scattered out into the black void of space. Hidden
among that trail of refuse, the Millennium Falcon tumbled undetected
off the side of the larger ship, and was left far behind as the
Avenger streaked away.
Safe at last, Han Solo thought.
The Millennium Falcon ignited its ion engines, and raced off through
the train of drifting space junk toward another system.
But concealed among that scattered debris was another ship.
Looking at various close-ups of the junk is no help, either. It appears to be a mixture of scrap and engine parts - possibly the "irreparable machinery" mentioned in the novelization.
However, we can speculate based on what we know:
Star Destroyers, in general, patrol the galaxy and often tractor in ships for inspections. When a ship owner is arrested, their ships are either auctioned off or stripped down. The official reason for this is to pay for the owner's legal fees. We also know from the Legends EU that this particular fleet of Star Destroyers has been travelling all over the hyperspace routes, arresting & interrogating anyone who might know where the Rebels (and particularly Luke Skywalker) are hiding out.
Based on this, we can speculate that the garbage contains at least three things:
- Large, compacted chunks of smaller garbage from the compactors
- Engine parts & machinery that cannot be repaired and is not worth recycling
- Stripped down frames and/or "corpses" of confiscated ships
If it's standard procedure to dump this stuff before going to lightspeed, then we have to assume that the debris has accumulated since the fleet jumped into the Hoth system - possibly the crushed remains of rebel ships. Of course, it's also possible that they skipped the procedure on the earlier jump since Vader was in a hurry.