The +7 means that team is “getting’’ seven points from their opponent. For example if the New York Jets are +7 against the New England Patriots than a bet on the Jets means you think the Jets will not lose by seven points, or more.
If you’re right you win your wager. The -7 is just the opposite. It means that team is “giving’’ seven points to its opponent.
Another example is if the Green Bay Packers are -7 against the San Francisco 49ers than a bet on the Packers means you think the Packers will beat the 49ers by more than seven points. If they do, then again you’re a winner.