The plus sign followed by a number is the number of goals a team is getting. For instance, in an English Premier League match if Arsenal is +1 1/2 vs. Manchester City then a wager on Arsenal means you think Arsenal is going to win the match outright, force a draw or only lose by one goal. If any of those three happen you win your wager. If Manchester City wins by two goals or more, however, you lose your wager.
To use the same example, if Manchester City is – 1 1/2 against Arsenal and you bet on Manchester City, they must win by two goals or more for you to collect on your bet. If City loses, draws or only wins by one goal, then you lose your bet.