This type of bet gives one team a handicap, meaning that that team needs to win by two or more goals for the bet to be successful. In a +1.5 handicap bet, the team that is given the handicap needs to win by two goals for the bet to be successful.
When you place a bet on "Over 1.5 goals," you are predicting that the total number of goals scored by both teams combined will be two or more. If two or more goals are scored in the match, your bet is considered a winner. If the match ends with one goal or no goals, your bet would be a losing one.