The + sign in front of a number for example, +350 means that a bet has been placed on the underdog, and the three digits following that sign indicate the amount paid out if the bet wins and the bettor had wagered $100.
For example, if the odds for a particular team to win a championship are +500, this means that a bettor would receive a payout of $500 for every $100 they wagered if that team goes on to win the championship.
