Bet is a slang term of affirmation, agreement, or approval along the lines of Cool!  or I'm down!  It can also suggest doubt or disbelief: Yeah, sure. 

Yes/No betting is simply betting for or against an outcome or statement. For example, Will Man United beat Liverpool? , and then the punter bets yes or no against the given odds.

noun. bet [noun] an act of betting. bet [noun] a sum of money betted. stake [noun] a sum of money risked in betting.