In the simplest terms, a negative spread indicates the favorite, which is the side expected to win the matchup. A negative point spread really means the team has some work to do. For a negative spread bet to hit, the team has to beat its opponent by a margin greater than the point spread.
The negative value -9 indicates the Giants are favored by 9 points. The positive value (+9) indicates the Jets are underdogs of 9 points. To place a bet on the favored Giants means they must win by at least 10 points to cover the spread. The underdog Jets can lose by eight points and still cover the spread.