The spread, also referred to as the line, is used to even the odds between two unevenly matched teams.
Bookmakers set a spread with the hopes of getting equal action on both sides of a game. For example, the Colts are a -3 point favorite against the Texans. The -3 points is the spread. If you want to bet the Colts on the spread, it would mean the Colts need to win by at least three points for you to win the bet. If the Colts win by two points, you would lose the bet because they didn't hit the key number of three.
The spread at -3 suggests the teams are fairly equal and maybe in this case, the Colts are only -3 because they are playing at home. Bookmakers see these teams as equal and expect to receive fairly even money from bettors. If these teams were widely considered equal and bookmakers put the spread at -10, they wouldn't get equal money as Texans +10 would likely be pounded by bettors.
In a spread bet, the odds are usually set at -110 on both sides, depending on the sportsbook and state. That means whether you bet the Colts -3 or Texans +3, you'll win the same amount of money if you win the bet.
If you like the Colts to win but think three points is a tricky number, a moneyline bet could be made on the winning team with no points in the equation. In the above example, a Colts moneyline bet doesn't have any other requirements other than they win the game. The difference is that their odds may be set at -140 to win. On the other end, the Texans could be +120 to win.
While betting the Colts moneyline leads to a smaller payout, an underdog moneyline bet on the Texans would yield more money because you'd get money on them winning the game straight up. In a spread bet, even if the Texans won straight up, you'd still only win the bet because they lost by less than three points.