Okay, I went and verified what I remembered about FAT filesystems since you wanted to understand what that meant but I did not want to just tell you what I remembered because I might be wrong
I couldn't find a concise non-technical version buried in piles of low level talk so here's my best shot...
In a FAT file system the directory has a table that contains things like filenames, file sizes, attributes, subdirectories and (most important to this thread) the location of the first cluster of each file.
The computer has the first cluster of where the root directory is hard coded in a specific location so it knows where to start.
Now you have a file in c:\movies\mymovie.mov. The computer finds this by looking at the root directory table and finds the entry for the directory MOVIES and looks at the first cluster value and finds it on the disk and its directory table. The MOVIES directory table has an entry for MYMOVIE.MOV with the first cluster of that file.
Lets say that MYMOVIE.MOV occupies 3 clusters and the directory table says that the first cluster is cluster 6 on the drive. How do you find the other clusters since they are not always next to each other?
I'm oversimplifying some here.
There is a map stored on the drive called a File Allocation Table (FAT) that you can think of as kind of a spreadsheet with a cell representing each cluster that contains a 32 bit number (FAT32). That number represents where the next cluster is or it is FFFFFFFF which means it is the last cluster.
Since you know the first cluster for MYMOVIE.MOV is 6, the computer looks up that cluster on the FAT and in the cell for cluster 6 the value is 7. That means the second cluster for the file is cluster 7 on the partition. When you look at the box for cluster 7 on the FAT it has the value 21 so the third cluster is cluster 21. When you look at the FAT entry for cluster 21 it is FFFFFFFF.
Now the computer knows to get the file by looking at the data stored in cluster 6 then cluster 7 then cluster 21.
So now you see the conundrum of losing the first cluster's value to corruption
how do you find the other clusters?
You can find pieces in the .chk files but it is very hard to put them back together for anything but text files because of fragmentation.
That's about the easiest way that I can put it.
Hope this helps.