A handicap of +1.5 means you can add 1.5 to a player’s total games in a match when determining the total score. So, if you bet on Nadal +1.5 against Federer -1.5, your bet would still win even if Nadal won 18 games while Federer won 19. This is because you add 1.5 to the total, so while Nadal lost the match, his handicap means the bet wins, regardless.