The Barrier of Knowledge is the veil through which you pass whenever you learn something. It's as simple as that, but it's so interesting.
Let's say I have a run-in with the Men in Black, and they wipe my memory completely-- my mind is now a true tabula rasa: I know nothing. From here on out, anything that I observe will teach me something brand new, and this information will become the context in which I ground my reality. Cool. But more interesting to me than the idea that I am shaped by the new things I learn is the idea that I cannot be unshaped by them. Sure, I can be reshaped, but never unshaped.
In other words, the present me is, in every passing moment, forfeiting the perspective of the former me.
Let's rewind to my elementary school years, when I was reading the Harry Potter books. If I'm on The Order of the Phoenix, and you walk up to me and tell me that Dumbledore dies in book six (I intentionally dropped that spoiler without warning you first-- if you haven't finished the Harry Potter series and you're sitting here reading my blog, you have some Sirius priority issues, and you should feel bad), then you've "ruined the story." We actually say that! Because you told me what happens before I made the full journey there, you ruined the experience for me. What you have done cannot be undone. It is the Barrier of Knowledge, and it's a one-way Veil (gee wiz, this post is Riddle-d with Harry P. references). You can say that you were kidding and try to convince me that everyone's favorite Gandalf doppelganger doesn't really die in The Half-Blood Prince, but it won't do any good. I'll still be expecting it when Snape casts the Killing Curse on Dumbledore. You can reshape me, but you can't unshape me.
Like I said before, once you pass through the Barrier of Knowledge, you forfeit the perspective of your former self. This can be an issue when it comes time to teach something, if you are unaware that the Barrier of Knowledge exists.
For example, if I told you that Euchre is a game in which ace is high, trump is higher, and right and left bauers are highest and that you need to take a majority of the tricks when you're in the barn to win, you might be a little confused. One cannot use the vernacular of a particular game to teach a new player the rules, yet sometimes it's difficult to remember that terms which become so familiar over time are completely foreign to someone who hasn't been exposed to them.
One of the reasons that I spend so much time thinking about this is because I don't think that very many of my professors are as aware of the Barrier as they should be. It's taken me years of exposure and countless hours of study just to speak the language of physics, and honestly, I'm still not there, yet. I'm not saying anything particularly complicated when I claim that it's a requirement for operators in Quantum Mechanics to have an orthonormal basis of eigenstates, I'm just not respecting the Barrier of Knowledge. Like, at all. I firmly believe that just about anyone can learn just about anything, but I also believe that it has to be explained in a way that is accessible to them.
In the summer, I work as a math instructor, and I see people on both sides of the Barrier of Knowledge struggling to send messages through it on a daily basis. But being aware that this barrier exists is the first step to training oneself to respect it and thus become a better communicator in all areas of life.
Using big words in everyday conversation? Probably not respecting the Barrier of Knowledge. Speaking gamer to a jock? Not respecting the Barrier of Knowledge. Explaining the perpendicular of a slope as the negative reciprocal to someone struggling with their Algebra homework? Not respecting the Barrier of Knowledge.
It's not a trap we mean to fall into; it's honestly just the result of lazy communication. Which is why we should train ourselves to speak accessibly, no matter the context-- to respect the Barrier of Knowledge.