It means they learned it by reading.
I saw this statement in a meme on Facebook today and it really struck home.
I've always been proud of the extent of my vocabulary and tried to use the words I knew as often as I could. I wasn't trying to be pretentious. It was because I loved words and how they fit together. That was the subject of my last post. It was an integral part of my need to write.
I think I read more than I conversed when I was younger - and maybe still do. But printed books don't come with pronunciation keys. So, I inevitably mispronounced words and did get laughed at when I misspoke. Some kids my age and even some adults accused me of being a show-off or a know-it-all and they offered my mistakes as proof. All that did was convince me that I should speak less and read more.
Reading is vitally important in all its shapes and forms. Books, newspapers, magazines, essays, memoirs, fiction, fantasy... It's how we learn language, of course, but there is so much more. There are lessons about society, social expectations, culture, history, science, the world, psychology, behavior patterns (and what seems acceptable and what isn't), problem-solving, self-worth, emotional strength or lack thereof, mental health, the meaning of love, the meaning of life. It can be an escape from reality or a return to the real world.
So if you hear someone mispronounce, first make sure the mistake is theirs and not yours. Then, correct them gently and graciously. Maybe ask them where they learned the word and how long ago. And, always always, encourge them to keep on reading.