Simply put, this type of bet gives the underdog team a slight advantage of 0.25 goals. This means that if the underdog team wins the game outright, the bettor wins their bet. Additionally, if the game ends in a draw, you only lose half of your stake.
A +1.5 spread is commonly seen in baseball betting, the standard runline for MLB. This spread means the underdog must win outright or lose by exactly one run to cover the spread. Alternatively, a -1.5 spread means that the favorite must win by at least two runs. Many baseball games are decided by fewer than two runs.