Typically, a short "story" joke has three parts -- the setup, the narrative, and the punchline. The setup sets the scene. For instance:
A man walks into a bar with a duck on his head.
The narrative describes the action that takes place after the scene has been set. To continue:
The bartender asks, "May I help you?"
The punchline supplies the humorous conclusion, of which there are several to this example and which I'll leave to others to supply. If the audience for the joke teller thinks the setup is amusing enough, they are said to "buy the premise," meaning that they are prepared to find the punchline absurd enough or incongruous enough or silly enough to laugh. If they do, they are said to "buy the bit."
An axiom attributed to Johnny Carson says, "If you buy the premise, you'll buy the bit." In other words, if the setup is amusing enough, people will be disposed to laugh at the punchline. But every comedian knows that a setup, no matter how amusing, isn't enough. A punchline must follow.
Here's Jeb's setup:
I was in Washington, Iowa, about three months ago, talking about how bad Washington, D.C. is
This is a potentially amusing contrast. Washington, Iowa is a place of absolutely no importance to anyone, probably not even to some of the seven thousand people who live there. Washington, DC, on the other hand, is a metropolis and the capital of the country. And people in Washington, Iowa and the audience for the debate are disposed to believe bad things about DC. So people may be prepared to laugh, but even people at Republican debates expect a punchline.
But Jeb didn't have one. And he didn't realize he needed one. And he expected laughter anyway. When he paused and didn't hear laughter, he haltingly tried to explain what was funny. I'll extrapolate:
It was [very funny] ... get the [joke?] ... [it was] kind of the [way I found myself in one Washington talking about another Washington]
If you have to explain your joke, it isn't funny. But Jeb tried anyway, apparently unaware that he had only 1/3 of a joke.
This was widely reported to illustrate just how clueless and humorless Jeb Bush is: First, he had only 1/3 of a possible joke and couldn't understand why it didn't get anyone to laugh. And secondly, he thought he might explain himself into the response he wanted.