In soccer, you can bet on the total number of goals that are scored during the match or in American Football you can bet on the total number of points scored in the game.
For example in an English Premier Soccer League match between Manchester United and Manchester City the total goals line might be 2 1/2. If you think the total number of goals would be three, or more, than bet the over; if you think the total score will two goals or less than bet the under.
If the final score of the match is Manchester City 2, Manchester United 1, then a bet of more than 2 1/2 goals is a winner. If the score is Manchester City 1, Manchester United 0, then a bet on less than 2 1/2 goals in a winner.