That would be a bet placed on the total score of a game to be under 52. Over/under bets are very common among bettors who wager on the NFL. The oddsmakers come up with a number for the total score of the game they feel is fair to both sides, and as they do with the point spread, a number the will generate equal money on both sides. In this instance that number was 52 points. And the U 52 meant someone bet on the under.