SubtractAsync is a variation of the UnionAsync function which allows you to create a union with a table of parts.
First…
I would recommend looking at the documentation, it can be extremely helpful and break it down in a simpler way.
However…
SubtractAsync essentially takes a main part and removes a table of other parts from that main part. You can imagine this is very taxing on roblox’s memory and even can take a short period of time to fully calculate if used in real time. I would HIGHLY recommend you to stray away from trying to use SubtractAsync in a destruction game unless your ideal audience is PC players only.
Finally…
I recommend using a Voxel Destruction Module, voxel destruction is the most optimized method of destruction I believe.