To vote for one politician (monkey) to prevent an even more primitive and under-qualified "monkey" from gaining the office.
The "Monkey Block Vote" is often used in elections when none of the candidates are fit for the office, but it's imperative to keep a complete idiot out by voting for the "lesser of two evils".
Say you hate both the candidates, but you hate candidate B more. You're going to vote for candidate A, even though you don't like him, to Monkey Block candidate B from gaining office.
Man, none of the candidates for the 2012 election are really fit for office. However, I gotta Monkey Block former governor Romney to keep him from making things worse.
Johnny has decided to be a Monkey Block Voter this election even though he doesn't think Obama is quite fit for office anymore.
Even though he isn't impressed with Obama's presidency so far, Johnny is going to vote for him to Monkey Block former governor Romney so the country doesn't get completely messed up.
Hey, I hate 'em both, but I just gotta cast a Monkey Block Vote because hopefully Romney can make things better, even though he isn't quite fit for the job either.