1.5+ odds come up in tennis handicap betting, and detail who is the favorite to win the match and who is the underdog. Tennis betting sites give ‘positive’ and ‘negative’ points in line with these expectations, and these show what the outcome of the match has to be for the bet to win.
If a player is assigned 1.5+, this means that they are the underdog in the match and must not lose by more than a two point score difference. If the difference is not more than two points, you will still win the bet.