Chunk

Debris comes in 2 types for now, and both stone and slag debris can be found scattered all over the map, it can be used as cover for colonists and raiders, stone debris can also be cut into stone bricks.
It takes longer to move over debris (how much? difference between stone and slag?) than moving on the ground or a floor.
if used correctly debris can slow the approach of raiders and give the colonists more time to fire at them, if used "wrongly" it will give cover to the raiders witch makes them harder to kill.

Debris can also be used to discourage the enemy's path finding algorithm from attacking a particular place or wall by making it easier/faster to reach the target through another way, than moving over all that debris that lies in the way.

Slag debris

No special uses more than whats mentioned above.

Stone debris

With a Stonecutter's table stone debris can be cut into Stone bricks.