The MID is the code that is stamped on the disc to allow DVD burners to know which writing strategy to use to get the best results.

This also allow users to find out who made the discs, in most cases, by using a DVD identification program like Nero CD/DVD Speed. Since brands tend to chop and change the manufacturer of the discs then the MID can be a big clue in the quality of the discs.

Unfortunately since the MID can be scanned very easily there are a number of manufacturers that add a MID that does not match the construction or quality of discs. This means there are a lot of 'fakes' out there so the MID isn't the be all and end all of disc identification.

You can use DVD Identifier to get info on your discs.