This article contains some useful tips on how to run queues and workers in production. The most important thing is that worker processes need to run all the time even if something goes wrong.
The command means that if there are jobs in the payments queue, these worker can only process them. However, if the payments queue is empty, they can pick up jobs from the notifications queue as well.
Now if the payments queue is empty this will happen: Now payments workers also pick up jobs from the notifications so we don't waste precious worker processes.