Specifically, over 1.5 refers to a bet on the total number of goals scored in a match. In soccer, for example, if you place an over 1.5 bet, it means you are predicting that there will be two or more goals scored in the match.
An over 1.5 bet is a bet that there will be at least two goals scored in a match. This means that if the match ends in a 1-0 win, the bet will lose. However, if the match ends in any other scoreline, the bet will win.