While looking at the board inside of a sportsbook you will notice a number above or below the line (sometimes it is also located in its own column labeled total). This number is the over or under number; it is usually between 40 and 70 for college football (NCAA FB), and between 30 and 60 for professional football (NFL). This number is the prediction for the total points scored during the game (both teams added together).
Over/Unders are an alternative to bet if you do not want to choose a team to win or lose either way. This way during the game you can root for points if you bet on the over, or lack of points if you bet on the under. Betting the over/under works the same as betting anything else of the board. There are odds assigned to each games total such as -180 or +220. The bettor has the option of taking the over/under as a straight bet or it can be added to a parlay. You can also do an entire ticket with just over/under bets.
Taking an over under off the board gives the bettor more options because they include most of the college games as well as the professional games. When betting over/unders on a weekly ticket (the rectangular papers where you bubble in picks like a scantron) the sportsbooks only offer over/unders for the professional games. The bettor also has the option to mix in over/under bets with bets on teams on one ticket. However, remember that adding more bets to a ticket decreases your odds of winning, but that also means more money.