what does reduced breaking power mean?

You have SBC brakes. It's an electronic break by wire system. There could be a malfunction with the SBC pump. If it's a red message you need to have the car towed in and do not drive it. If it's a white message it's still important to get it diagnosed with a professional scanner but you can drive it in. Keep in mind Mercedes just extended the warranty on the pumps for 25 more years so don't pay for a new pump. If that's what's wrong with it the dealer will do the job for free provided you have performed brake fluid flushes on time. Alex