a checksum is an algorithm applied to a file (or cd) which when completed cretes a small string which is unique to that file.
So, if you download an importand file (like the operating system) its a good idea to run a cheksum check on it.
If the checksum of the downloaded file mathes exactly the checksum of the file on the server, then the download was a complete sucess. if however the checksum does not match, then the file was corrupted on download.
you can probably check disk 6 by getting its checksum string from the distributor.
If the checksum check fails, then your coppies are corrupt, and you should get a new copy.. either buy a distribution (you can get them very cheap from ebay, (it is perfectly legal to make coppies of linux, and RE-SELL them) or re-download the file.
maybe your disks will still work, its a gamble and might cause crashes, or whatever, you dont know. maybe the corrupted part is just a screen saver... maybe its a major runtime library, or worse the kernel