A point spread bet is a wager on which team/player will win the match/game according to a point spread. A point spread is a number created by oddsmakers to determine the gap between the favorite and the underdog of a game, as teams and players are not all equal in skill. The favorite will have negative points while the underdog will have positive points.
If you're taking the points with the underdog, the points will be added to your team's final score to determine the winning outcome of your bet. If you're laying the points with the favorite, the points will be subtracted from your team's final score to determine the outcome of your bet. If the score ends in a tie after your points are added/subtracted the bet pushes and your wager is returned.
Using the below example for the NFL, the Buccaneers are the favorite at -275 moneyline while the Cowboys are the underdog at +220 moneyline odds. In order to even out the gap between the favorite and the underdog, a point spread of 6 points is applied so that both teams have odds of -110. Since the Buccaneers are the favorite, they will have -6 or 6 points removed from their final score. Since the Cowboys are the underdog, they will have +6 or 6 points added to their final score. For example, if the final score of this game ends up Buccaneers 24, Cowboys 20, bettors that bet the Buccaneers -6 lose the bet because 6 points is subtracted from the Buccaneers final score which would make it Buccaneers 18, Cowboys 20. The reverse would be true for bettors that bet the Cowboys +6. Bettors that bet the Cowboys +6 would win their wager as 6 points is added to the Cowboys final score which would make it Buccaneers 24, Cowboys 26.