Some things are things you know about. For example, if you are an accountant you know about some kinds of financial documents, they are well known things to you. You surely have other things that is known unknown. If we use an example with financial documents, there are certain documents that an accountant has only vague idea about. I can also say that for known unknowns you have only partial information.
There are also yet other things, unknown knowns. You know about them, you have them in the long-term memory, but you can’t remember them right now. Most likely you need a specific cue to create a link to these memories. These things may be related to topics you haven’t discussed for a while, or it may be something related to another context, not the one you have right now.
And the most distant from one’s mind is the last category, unknown unknowns. Basically unknown unknowns are things you don’t about. You most likely don’t know much about the area, and don’t know about related things. For example 150 years ago people didn’t know about radioactive decay, so for everyone it was an unknown unknown. People might become ill from ionizing radiation, but as they could only blame other causes.
Even in present days many people don’t know about radon. Of course some people may know, for example if they are construction workers or engineers. Radon is a colourless, odourless, tasteless radioactive gas. It is produced by the radioactive decay of radium, which exists in small amounts in soil. So underground tunnels and ground floors of buildings may be contaminated. And due to its radioactivity radon may induce cancer. Until you knew about it, radon was an unknown unknown, but now it is a known unknown. You know that it exists and that some living and working places may have it. If you measure radon concentration in a specific place, it will become known known one.
Right now you may think something like “it is impossible to know everything, to eliminate all unknown unknowns.” That is correct, but you can look for unknown unknowns in areas important for you. One of good methods is looking for alternatives: you should consider alternative ideas, theories and explanations of anything. It can help you to expand your knowledge and transfer important unknown unknowns into known unknowns.