A sportsbook is the same thing as a bookmaker or bookie: It's a company or individual that accepts bets from individual sports bettors. Sportsbooks accept bets on either side of a sporting event. They're able to afford to do this because of the difference between what a bettor has to wager and what a bettor wins.
The underdog\n\n A plus sign (+) in front of the handicap indicates an underdog. The 49ers are the +1.5 underdog in this game, meaning oddsmakers believe they will lose, but only by a point. To win this point spread bet with the 49ers, San Francisco would need to win the game outright or lose by one point exactly.