The term literally means:
There is no truth to the rumor that it refers to the writer's frustrated creation of a new swear word.. or that it's the sound of a psychosomatic induced sneezing from Writers-Blockitis.
It means just like it sounds.. get your butt in the chair, put your hands on the keyboard, and DO SOMETHING.
The old adage is that if you give enough monkeys enough time plunking away on a typewriter.. eventually you'll reproduce the entire repertoire of Shakespeare. I don't know if that's true or not, but it seems probable that they'd at least stumble across something pretty close to some of the crap that gets published these days.
But.. here's the kicker to that scenario: The monkeys have to actually be plunking away on a keyboard before they can reproduce any of old Will's plays.
So, if a Monkey can do it.. then we should be able to do something similar, afterall.. scientists tell us we share 93% of the same genetic makeup as our simian friends..
.. and I don't know about you guys, but I'd be happy sharing 93% of their level of writing success if they're reproducing Shakespeare.
Hell, I like those odds.