I think the abbreviation comes from Deliberately Meaningless Index but it is pretty good indicator to show how well your players are going to perform. I'm not saying it is the only thing you should focus on but it really gives pretty good information of your players and you can scout your opponents which can be even more significant.
The main thing in this is to focus on your gameshapes. The better your gameshape is the better your players will perform and this shows directly in your players' ratings in the games. All of this info can be read via the link others have already posted.
For example if your players gameshape is respectable (7) for $ 20k player, his DMI is something like 70k (3,5 times player's salary). When the gameshape is strong (8) for the same player, his DMI will be something like 180k (9 times player's salary) and with with proficient (9) gameshape it will be something like 250k (12 times player's salary). Then again if player's gameshape drops to average (6), his DMI will drop to about 60k (3 times the salary).
In gameshape indicator there are sublevels, so it will not be exact but still gives you some info of the player: salaries are calculated prior every season, so players can be trained and after season their salaries will be higher because the are improved. There's no other ways to scout your opponent's players current skills if they are not for sale. Your own players future salaries can be calculated with numerous tools made for BB.
Try to give each of your players 50-about 80 minutes weekly, so your gameshape is about to increase. There's some randomness is gameshape, so even if you give your players exactly right minutes still gameshapes can be the same or even drop if they are already high. Then again if you overplay your players or you're not going to play them, their gameshapes will drop and they are not performing as well as they could. When playing in lower leagues you can easily beat your opponents even with weaker team if you manage to take care of GS.