Betting on the total means that you are betting on the total amount of points scored by both teams combined.
There are two ways to bet a total line - the over and the under. Bet the over and you'll win if the teams combine for more points than the lines states; bet the under and you win if the teams combine for fewer points.
If the total combined score falls exactly on the number you bet, the wager is a push.
NOTE: on certain leagues and games there are proposition bets offered on individual team totals, as well as totals for the first and second halves of the game. These bet types will always be clearly labeled to tell them apart from each other.