Over/Under is a popular betting option as there are only two possible outcomes and so the overround of the bookmaker is much lower. Over/Under simply means that you can bet on "over a certain number" or "under a certain number".In soccer for example that certain number usually (but not all the time) is 2.5 and it represent the goals that are to be scored in that match.In baseball it's around 8.5 and it stands for the number of runs; adn so on with all the other sports.You may ask how can there be 2.5 goals in a soccer match.There can't be.But this decimal number is given for the simple fact that under 2.5 can only represent 0, 1 or 2 goals while over 2.5 represents 3 or more goals;and so thing are much easier and much more clear.