For example, the over/under between the Los Angeles Lakers and Boston Celtics is 230.5 points. If you think it will finish with more than 230 points, you can bet the over. If you think those teams combine for less than 231 points, you bet the under.
The most common type of over-under bet is on the combined score of two teams in a match. In this case, a bettor will wager that the total number of points scored in a game will be higher or lower than a set value (normally referred to as the over-under for that game). If they are correct, they will win.