A point spread handicaps the favourite by a set margin — lay -6.5 points and they must win by seven. Both sides are priced near -110, the book's profit living in the vig rather than in shaded win odds. Whole-number spreads can land exactly (a push) and refund stakes; half-point lines cannot.
It is the same idea as the Asian handicap in football: convert a mismatch into a coin flip and charge for the conversion. The football-specific quirk is that low scoring makes half-goal lines dominant, where US sports live on 3.5s and 6.5s.
Favourite -3 at -110: win by exactly 3 pushes (stake back), by 4+ wins, by less loses. The -110 on both sides means you must beat the spread 52.4% of the time just to break even — the number every 'against the spread' record should be judged against.