"...there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns—the ones we don't know we don't know." - Donald Rumsfeld
Basically if you know that something exists, but don't know about it you can still go learn it and apply it to solve a problem.
But if you don't know about it, most likely you'll try to reinvent the wheel and spend way too much time doing it.
It always annoys me when that phrase is attributed to Donald Rumsfeld. The concept is much older than him, and the attribution to him gives him an air of wisdom that he doesn't deserve.
Basically if you know that something exists, but don't know about it you can still go learn it and apply it to solve a problem.
But if you don't know about it, most likely you'll try to reinvent the wheel and spend way too much time doing it.