Hi!
In short, this determines the scope of the selected thread count. When running a multi-instance environment (for example, with 4 containers), you could choose a thread limit of 4 per node (which makes 16 in total) or cluster (4 in total).
You can read more in the documentation here; https://docs.mendix.com/refguide/task-queue/#create-queue
Hopefully, this helps!