What' does the "checking disk" button do?

Ok, so i tried restarting my PC and it showed a "press any button to skip checking discks" (or something like that?)

i didn't see this in time and it started "checking D://" or something like that...with like 8 hours proceeding time?????

What is this?? Should I do that and let the PC do that job for 7-8 hours or? Is there an issue with my PC? what's going on?