A handicap gives one side a virtual head start before kickoff. Back the favourite at −1.5 and they must win by two or more; back the underdog at +1.5 and they pay out even in a one-goal defeat.
Half-goal handicaps always settle. Whole-goal ("European" vs "Asian" conventions differ here) can void the bet or split the stake when the handicap lands exactly.
Handicaps exist because lopsided 1X2 prices are unbettable — a 1.10 favourite carries huge risk for tiny reward. The handicap rebuilds the matchup into a roughly even proposition, which is where prices are most honest and analysis matters most.