I have a simple question, what exactly is ''the spread''?

Bookie: The spread or point spread is the predicted scoring differential between two opponents.A wager on the favored team (a  "-" indicates the favored team) means that this team must not only win, but also win by the specified point spread. A wager on the underdog (a "+" or no text next to the team indicates the underdog team) means that this team may either win or lose as long as they do not lose by the specified point spread.

If you chose the favorite at –5, this means that team has to win by more than 5 points to win. If your team wins by 5 points you push. However, if they win by 4 points or less, you lose. It’s that simple.