This is a quote from science fiction GRAND MASTER, Robert Heinlein, possibly & ironically most famous for in recent pop culture, as the original source of the Hollywood dud, STARSHIP TROOPERS, a book he penned in 1959. (this is also one hell of a dud sentence, but it's my fucking website, so kindly deal with it)
Most people react to the last sentence in Heinlein's quote, which forgoes context. Given at face value, its a cry against our Western ideal of mastery, and an endorsement of the "Jake of All Trades" pejorative . But in context, both as a quote and in the world we are living in, I think it's a very critical thought. Specialization is an industrial desire, and in our idea centric world, a sure way to be locked into a pivot-less occupation, when the world is accelerating exponentially.
I am a propionate of mastery. But, mastery in it, and only of itself is akin to obsessive compulsiveness. It's a limit of human potential, and of living an interesting and fulfilling life. We should all master one thing, but we should all carry that curiosity and drive into other things that are deserving of our attention. This well rounded pursuit is not to be at the top 5%, or 1% best at all things, but to be capable human beings well suited for adaption. Human beings with ability, empathy, & drive.
So, learn how to code, play an instrument, build a table, or pick up a chemistry book because it is through these other things that you can become indispensable on a higher level at the one thing that really gets your blood boiling. Ultimately, learning is the great human gift.