MDS: Multiple Displacement System.
In the 5.7's and 6.4's w/Automatic transmission, it is a system that disables 4 of the 8 cylinders when they are absolutely not needed. Turns them back on within a split second when they are needed such as slamming the gas pedal down. So when you are cruising on an interstate at 65 with or without cruise control on, you are actually running on 4 cylinders. Slam the gas pedal to pass a trucker going 55 or a granny, and within that split second or less, all 8 cylinders are kicking.
It's purpose to improve fuel economy. Which in my opinion it successfully does so and I leave it on. Dodge/Chrysler state that the system saved overall millions of gallons of gas.