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.
An acca is a group of individual bets brought together in one mega wager. Each leg of your acca, or accumulator, must win in order for you to win the overall bet and get a payout.