One number will be given, such as '47 2 for an NFL game total. You can then either bet the 'over' or the 'under' and it's exactly as it sounds. If you bet the over, you want there to be more than 47 combined points scored, and if you bet the under you want there to be fewer than 47.
In basketball, if you place an over 1.5 bet, it means you are predicting that there will be two or more points scored in the game. In baseball, if you place an over 1.5 bet, it means you are predicting that there will be two or more runs scored in the game.