A "mug bet" is a technique used by matched betters to keep their accounts with bookmakers open. It is usually a small bet that emulates a normal "punters" betting activity.
Accumulators, regular bets for the same teams, spending money in the online casino, putting bets on high publicity events, all improve your profile in the eyes of bookmakers. This makes it less likely your account will be closed down.
Depending on the value of your mug bets, you are encouraged to lay them off so you can minimize any potential loses. Remember we're not in the business of gambling here!