When you place a handicap bet ("+" or "-"), the team/player you bet on will have an advantage/disadvantage from the start, depending on the type of handicap chosen. In order for the bet to be a winning bet, the team/player must score more goals/points in order to get the desired result with the chosen advantage or disadvantage.
Here is an example: "Manchester United -1" means that the team starts with a handicap of one goal. For the bet to win, Manchester United must win by a two-goal margin. Basically, the handicap is added to the score at the end of the match and the final result must be in favor of the team you bet on.
On the other hand, if you bet on "Manchester United +1", the handicap works in their favor and they start the match with a one goal advantage. For your bet to be a winner, Manchester United can either win the match outright or draw as the handicap gives them an extra goal on the scoreboard.