Essentially, gubbing is the act of having your account limited by a bookmaker. You'll get an email from the bookmaker stating that you have in fact been gubbed. Their email won't mention this term, but in amongst all of their jargon the same message will be passed across.
Gubbing can also affect your ability to place bets at normal odds, as some bookmakers may impose stake restrictions or reduce your betting limits. Gubbing happens because bookmakers want to protect their profits and avoid losing money to smart bettors who exploit their offers and take value from them.