Identifying a test in Disk Utility First Aid

When I run DU and click on First Aid, the test below (last one in the window, where it says: "Performing...") comes up. It seems to take forever and I have to go off and leave the Mac for probably at least an hour before it completes this test. Sometimes longer. During that time I can't do anything apart from force a restart, which I don't think is a good idea. It does eventually complete the process, but I was wondering if anyone could explain in layman's terms what that test is and why it takes so long.

My Mac is a MacBook Pro 13", running Mojave 10.14.5.