The under bet is even money, meaning a winning bet would double the amount placed on the bet.However, the over is -120, meaning you'd have to betR$120 to winR$100 (orR$12 to winR$10). This usually means that the over is getting more bets and/or is viewed as the slightly more likely outcome.
An over/under bet is determined by the total points/runs/goals scored by both teams in a game. The bettor selects either the "over" option (predicting more total points/runs/goals) or the "under" option (predicting fewer total points/runs/goals) compared to the specified game value.