The term bet can be used in a few different ways, but generally means "agreed" or "okay". For example, if someone says "Wanna grab lunch later?" and the other person responds "Bet", it means they agree to the plan. Bet" can also be used as a response to confirm that a statement is true.
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.