A player who is at -1 is one vote away from being executed. I bet you can figure out what -2 and -3 mean.
Many players will avoid putting a vote candidate at -1 if they want more information from that candidate. Once someone is at -1 it's easy for another player to hammer them and end the day prematurely.
Some players apparently refer to this condition as "L-1" but I have never met these people and I bet they're awful.