What does O/U or ‘Over/Under’ mean?

In "Over/Under" (Totals) it is possible to bet on the number of a specific event (e.g. goals, points, corners, rebounds, penalty minutes, ...).
 
Example: Club Brugge – Anderlecht –’Club Brugge over 2.5 goals’. This means Club Brugge will need to score at least three goals for the bet to be won.