What's the message of this Yogi Berra quote?

There are some people who, if they don't already know, you can't tell 'em.

- Yogi Berra

Does it mean you don't know how good or bad some people are unless you get too close to them? (i.e., you can't tell them apart)

Or does it mean, a secret cannot be revealed to some people no matter what?

It means that some people are unteachable: so dense or so close-minded that they are incapable of comprehending or accepting what you tell them unless it conforms to what they already know or believe.

Incidentally, this Yogi-ism is often attributed to Louis Armstrong, probably through confusion with what Satchmo is said to have told somebody who asked him to define jazz: "If you have to ask nobody can explain it to you." And that story, which starts cropping up in the 1950s, seems to derive from a line attributed to Fats Waller at least as early as 1938: "If you gotta ask what [swing] is, please don't mess with it."

StoneyB has provided what I take to be the correct interpretation of the quotation. To his answer, I would only add that many of the remarks attributed to Yogi Berra as "Yogi-isms" have in common a kind of logical turmoil that prevents the statement from making sense if taken literally. For example, Berra is also noted for having said:

Nobody goes there anymore—it's too crowded.

and

I wish I had an answer to that because I'm tired of answering it.

and of course

I never said most of the things I said.

The logical impasse in the OP's quotation is that, if certain people really couldn't ever be told (that is, learn) anything they didn't already know, they couldn't learn anything, period. What you already know from the outset, you don't learn; and what you learn along the way, you didn't already know. So the literal logic of Berra's remark is fractured, but the intended sense of it is, as StoneyB says, that some people are very bad learners because they are extremely resistant to new ideas.