If something is unknown, it's not familiar understood, like the unknown path through the woods you've never explored or your brother's unknown reasons for staying late after school.
When you're aware ofor familiar with something, you can describe it as known. Add the prefix un-, or "not," and you get unknown. You'll often find this adjective in the phrase "unknown territory," as in "I totally understand algebra, but calculus is unknown territory." And as a noun, the unknown sometimes represents a whole category of mysterious things: "Our fear of the unknown keeps us from traveling to new places."