Where does the stonith resource run, and what does it do?

So, the course material (page 402 of SLES15 lecture PDF) says:
“STONITH runs as a cluster resource on every node”
However, in every real life SLES HA cluster I have seen, there is just one stonith-sbd resource, only on one node at a time.
Also, I believe that the sbd.service is started and stopped by pacemaker.service. I have no idea what the stonith-sbd resource actually does.