Example: In education it's important for students to learn a wide range of things about a subject, even things they didn't know that they needed. This is why self-taught students often have large but important holes in their knowledge; they simply didn't know what they had to study, so they didn't look for it.
If you know that you don't know something, that's a "known unknown", and you can plan to research it. If you don't know that you don't know something, then that's an unknown unknown, and that's a much harder problem to fix without external help or accidental discovery.
> The aide said that guys like me were "in what we call the reality-based community," which he defined as people who "believe that solutions emerge from your judicious study of discernible reality." ... "That's not the way the world really works anymore," he continued. "We're an empire now, and when we act, we create our own reality. And while you're studying that reality—judiciously, as you will—we'll act again, creating other new realities, which you can study too, and that's how things will sort out. We're history's actors…and you, all of you, will be left to just study what we do."