What does "BMS" stand for on the LCD?

My battery generally displays a number in the "50.x" range. I realize a fully charged 48volt battery will display in the 50 volt range. But I have seen the number go down and then go up. Does this mean that the bike has regenerative braking, adding a small bit of charge back to the battery while being used? Also, what does "BMS" stand for on the LCD?