This is a label that is found on profiles that engage in two forms of cheating. Both forms are outlined in Lichess' TOS (http://en.lichess.org/terms-of-service
Sandbagging is the act of "deliberately playing below one's actual ability in order to fool opponents into accepting higher stakes bets, or to lower one's competitive rating in order to play in a future event with a higher handicap and consequently have a better chance to win" (en.wikipedia.org/wiki/Sandbagging
Essentially, if you attempt/aim to lose in order to lower your rating, you are sandbagging.
Boosting is the act of artificially increasing your rating. This can be done in many ways. One of the common ways is to create another account and play rated games against it. Another common way to boost one's rating is to ask a friend to help you do so. They lose, you win, and the computer slaps both your profiles with that red label.
You can be considered a booster if you assist sandbaggers. If you know that somebody is sandbagging and you continue to play them, you are artificially boosting your rating. It is tempting to continue, but please don't. The computer has no mercy and you will both be marked.
If you notice someone sandbagging, please do report it. Sandbagging affects many users and makes it less fun to play. (Example, if a 1200 plays at a grandmaster level, you would be pretty angry once you lose 35 rating points to the sandbagger.)
Thank you :)