Point spread
A handicap applied to the final margin so that a favourite must win by more than the line for the bet to win.
A point spread adjusts the result by a fixed margin. In a market of Team A -6.5 and Team B +6.5, you add the spread to the score of the side you back and see who is ahead. American football and basketball use it as the main market; soccer has Asian handicap; ice hockey has the puck line; tennis has game handicaps. The structure is identical.
Worked example: tennis game handicap
Illustrative: Player A -4.5 games at 1.90.
Player A wins 6-3, 6-4. Games won: 12 to 7. Adjusted score: 12 minus 4.5 = 7.5 against 7. The bet wins. A €50 stake returns €95.
Had it finished 6-4, 7-5, the games would be 13 to 9, a margin of 4, and the bet loses (13 minus 4.5 = 8.5 against 9).
Whole numbers and pushes
A line of -4 rather than -4.5 creates a third outcome. If the margin lands exactly on 4, the bet is a push and the stake is returned. Bookmakers price that possibility in, so -4 trades at shorter odds than -4.5 would.
Break-even
At the standard 1.91 on both sides (-110), your break-even win rate is 1/1.91 = 52.4%. Spread betting at standard juice is therefore a test of whether you can beat 52.4% over a large sample, not 50%. The difference is the vig.