A point spread is a handicap that makes two unequal teams roughly even. If a team is listed at -6.5, it must win by seven or more for a bet on it to pay. The other side, at +6.5, pays if that team wins or loses by six or fewer.
Spreads move when money or news arrives. A key injury, weather, or heavy betting on one side can shift a line by a point or more before kick-off. The number you see when you place the bet is the one that counts.
Most spread bets are priced near -110 on each side, meaning you stake $110 to win $100. The extra $10 is the sportsbook's margin. A half-point spread (such as 6.5) removes the chance of a tie; a whole number can produce a push, where stakes are returned.
Source: Playfield Review explainer, based on standard sportsbook pricing conventions.