I was just looking at the Stack Exchange leaderboards, and noticed that I have negative "change". However, I have definitely not lost 808 reputation this quarter. What does this value mean?
The number describes your change in position on the list since the last time the userbase was ranked for that period. In this case you dropped 808 positions from the previous quarter.
You are at position #3255, which is 808 places lower than last quarter.
Notice that 3255 - 808 = 2447, and that's where you were last month: