Betting Against The Spread (ATS) is betting a team to win by at least a certain margin or not lose by more than that amount. Let’s look at the same NFL game.
Example: Eagles -2.5
A bet on the Eagles -2.5 means the Eagles need to win by at least 3 points for them to “cover” and you to win the bet. Once the game is over, the Eagles have 2.5 points removed from their score, and then the team with the most points wins.
Example: 49ers +2.5
A bet on the 49ers +2.5 means the 49ers need to win the game or, lose by 2 points or less for them to “cover” and you to win the bet. Once the game is over, the 49ers have 2.5 points added to their score, and then the team with the most points wins.
The odds on spread bets are often -110 or close to it. That means you need to bet $110 to win $100. In the example above it is -120/-102. Betting against the spread is betting on the margin of victory. Not every sport uses the term spread, but the concept is the same.
The NHL and MLB default puck line and run line is +1.5/-1.5. In the NBA and NFL it varies a lot more and can be as low as 1.5 or significantly higher, like in the Bucs/Cowboys game where it's 6.5.