Debris comes in 2 forms for now: Steel Slag Chunks 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 Debris a somewhat better form of cover than bushes, but significantly better than trees (25%), but worse than sandbags (65%).
As a low-lying object; any pawn that passes over either form of debris will have slower movement as they 'climb' onto the debris, 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 forms of debris 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 cheaply build durable structures, and Steel Slag Chunks can be smelted at an electric smelter to make 10 pieces of usable steel.
Debris can also be used to discourage enemies 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.
Steel Slag Chunks are currently the only existing form of slag in the game. They are spawned when structures made out of steel are destroyed (excluding doors and walls), and also from drop pods and cargo pods as they smash upon impact. This can include enemies landing in drop pods, when you start by drop pods (e.g. crashlanded, rich explorer), or enemy sieges.
They can be smelted down for steel at an electric smelter.
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.
They can be made into stone blocks at a stonecutter's table.