They are American money line odds; for example, +200 signifies the amount a bettor could win if wagering $100. If the bet works out, the player would receive a total payout of $300 ($200 net profit + $100 initial stake).
The term bet can be used in a few different ways on social media but generally means agreed or okay. For instance, someone might say, Want to grab lunch later? and the other person responds, Bet . It can also be used as a shorthand to mean you can count on it or trust me.