As opposed to betting the point spread (a forecast of the number of points by which a stronger team is expected to defeat a weaker one), the money line is a bet made on a team to win the game straight up, with no points.
The money line favorite is the team that both the people that set that lines, and the bettors, think should win the game.
For example, when Super Bowl 53 (pitting the New England Patriots against the Los Angeles Rams) kicked off, the Patriots were -150 money line favorites. That meant if you wanted to bet that New England would win the game outright, you would have to bet $150 to return $250 back, your initial stake plus $100.
If you bet the Rams on the money line, you would have gotten odds of +135 ($100 bet would have returned $235). But you also would have lost your bet; the Patriots won the game 13-3.
Money line bets are very good if you think that the underdog is going to win the game outright, because you will win more money. In the 2018 NFL season, there was one game that was the biggest money line win since 1995. The Buffalo Bills stunned the Minnesota Vikings 27-6, at money line odds of +1000, or 10-to-1. A $100 bet on the Bills returned $1100.