The sportsbooks establish point spread lines for each matchup to try to draw relatively equal betting action on each team.
The spread is the number of points that the favorite needs to win by to cover. So, this means if you bet on the 76ers at -3.5, the Sixers have to win by 4 points or more to cover the spread. When you place your bet, you are picking which team will cover the point spread listed by the sportsbook, not which team will win.