(Note: when I'm using the word "idea" here, I'm using it in roughly the sense of "notion of a pattern that unifies many disparate observations", rather than "plan for achieving a goal” — in the sense of “do you get the idea?”, not “I have an idea!”).
Imagine two Canadians driving across the United States without a map, communicating their experiences to one another with flip phones as they build a mental image of the country. Let’s call them Graham and Bert — these are Canadian names, right? Right.
Suppose that Graham's wandering in the southwest, cataloguing all the different kinds of rocks out of sheer boredom, when he chances upon a massive shining city in the desert — Las Vegas. He immediately pulls out his flip phone and yells "Bert! Bert!! Get over here!"
But neither of their silly little flip phones have a GPS, nor do they have any maps — because of this, Graham necessarily has to recount his own experiences to Bert in order for Bert to get to Las Vegas. The process will be rough:
"Go to the southwest, drive past most of those mountains you’ll find, then, uh... there's a... weird road...?"
Having just gotten there along some series of unremarkable two-lane roads of which there are far too many to remember, he won't be able to tell Bert how to get there unless he drives this route over and over. If he gets there along a major highway like the I-15, however, he has a much wider range of explanations he can give, as he can just say something like
"Get to this general area, then find the I-15 — can’t miss it — taking it south until you reach this exit, at which point just keep driving until you see the huge city in the desert. If you hit Mexico on the I-15, you were probably already south of the city, so turn around and go north until you reach the city".
If, over time, Graham develops a mental map of the entire surrounding area, then not only can he give the highway route for someone far away, but he can give a much quicker local route for someone already pretty close:
“Go left, head ten miles along the bendy road — you'll pass this Elvis-themed rest stop on the way — then go right, merge onto the next road, ...".
This is more or less how we communicate ideas to one another.
Someone who has an epiphany will, if they reached it in a convoluted way, usually stumble and stutter and totally fail to explain it properly, but if they’ve reached it in a straightforward way, say by developing some simple idea in a certain direction/with a certain intention, they should be able to explain the epiphany to others with far greater precision by either focusing their exposition on on the idea and direction; if they’ve made themselves very familiar with the ideas near that epiphany, forming a detailed picture of the kinds of things the epiphany is about, they’ll also be able to explain it by walking through a precise series of mental movements.
The most noticeable differences between the roads-between-cities situation and the explanations-of-ideas situation are that:
Nevertheless, the metaphor manages to succinctly capture lots of patterns of that occur in the development and communciation of ideas. If we take the metaphor seriously, it tells us the following: There is a sense in which ideas exist independently of our cognization of them by virtue of their situation in a massive web of ideas and movements between ideas.
In this sense, our role is primarily to use our cognitive faculties to discover these ideas and the connections between them, i.e. to traverse this realm of ideas. Just as roads exist even if nobody drives them, and cities exist even if nobody visits them, we can take the points and paths of this web to exist in themselves, waiting only for a thinker to reproduce them. The “we can take” here is important — I’m not making the ontological claim that ideas actually exist independent of us, just the claim that it seems like we can talk as if they did.