Chunk
Chunks, also called Debris, comes in two forms: Steel slag chunk and various rock chunks, which can both be used to make usable resources. They can also be used as low-cover, with both variants providing a cover effectiveness of 50%. This makes chunks significantly better than bushes or trees (25%), but worse than sandbags (57%).
As a low-lying object, any pawn that passes over them will have slower movement as they 'climb' onto the chunk, but they won't move much slower after climbing as they run along it - this makes large clumps of debris less effective as to hinder enemy movement. Both types of chunk can be made into resources too: rock chunks can be cut at a stonecutter's table to make 20 stone blocks per chunk, which can be used to build durable structures, and steel slag chunks can be smelted at an electric smelter to make 15 pieces of usable steel.
By strategic use of (temporary) stockpiles, chunks can also be used to discourage enemies from attacking a particular place or wall by making it easier/faster to reach the target via a different route, rather than moving over all the chunks that you put in the way. A pawn has to be behind cover to make use of it, so a field of solid chunks only offers cover where it starts.
Rock chunks
Rock chunks can be found randomly scattered in various clusters generated across the map, with rock types corresponding to the rock types available on your landing spot. A rock chunk will also randomly be spawned with a thudding noise when mining rocks - the chunk corresponding to the rock it came from.
Rock chunks can be made into stone blocks at a stonecutter's table.
Slag chunk
Steel slag chunks drop when steel structures are destroyed (excluding doors and walls), and from crashed drop pods / cargo pods upon impact.
They can be smelted for steel at an electric smelter.