In most sporting events, one team is typically better than the other. Picking the better team to win, for many games, would be too easy and wouldn't offer the bettor a large enough payout. The Spread or “handicap” exists to even the playing field, and by doing so makes picking the outcome of the bet more difficult. Each outcome has a + or a – handicap applied to it; the underdog is given points (+) while the favorite gives points (-). The handicap is often referred to as the ‘line’.
Example: The spread for a given basketball game is 6.5 points. The 76ers are the favorite (-6.5) and the Celtics are the underdog (+6.5). The 76ers win the game by a final score of 100-93. If the bettor placed a wager on the 76ers (-6.5), they would win their bet, as the 76ers won by more than the determined spread (6.5). If the bettor placed a wager on the Celtics (+6.5), they would lose their bet as the Celtics lost by more than the determined spread (6.5).