Yellowfin can run 5 background tasks, so essentially 5 threads at once across different threads (scheduled tasks).
For example, broadcasting a report will run on a single thread, so if you are sending the report to 5 recipients it will run the reports 5 times on a single thread.
Other things such as scheduled cached filters will run across different threads, e.g. a single filter will run on 1 core, but Yellowfin can run each of the 5 filters at a time, essentially running 5 threads across 5 cores. The interface is multi-threaded, e.g. multiple users logging in at one will split across cores.