A Cognitive Wall is an intellectual wall between you and an idea that prevents you from connecting with/understanding the idea.
We implement a cognitive wall for emotional reasons, like low self-esteem (we erect the wall because we are afraid that if we try to understand, we won't), avoidance of painful awareness (we don't want to read a book on pedophiles because we are one, or we know one), because we reject something outright (atheists have a cognitive wall when it comes to discussions of God), or because we are indoctrinated to reject (Christians are indoctrinated to reject the tarot), etc.
You will know you have a cognitive wall if you pick up a normal book and simply cannot access the meaning of the words on the page. You will see the words, you will read them, but you will not be able to understand them, even when they are simple and straightforward. Cognitively, you will withdraw to another space. As you drift away, the words will lose all meaning.