Assembly language msdos source code equation between cluster and FAT

Question

Going through old MS dos source code on github, I found some peculiar comments as shown below.

;Now we must load the FAT from the disk. Here's how we    
;find out where it starts: 

;FAT cluster 0 = media descriptor = 0F0h
;FAT cluster 1 = filler cluster = 0FFh
;Cluster start = ((cluster number) - 2) * SectorsPerCluster 
+ (start of user) = (cluster number) + 31

Can anyone explain these?