“…if you aren't certain that you understand and can value your business far better than Mr. Market, you don't belong in the game…”

The full quote is

=======.

But, like Cinderella at the ball, you must heed one warning or everything will turn into pumpkins and mice: Mr. Market is there to serve you, not to guide you. It is his pocketbook, not his wisdom, that you will find useful. If he shows up some day in a particularly foolish mood, you are free to either ignore him or to take advantage of him, but it will be disastrous if you fall under his influence.

Indeed, if you aren't certain that you understand and can value your business far better than Mr. Market, you don't belong in the game. As they say in poker, "If you've been in the game 30 minutes and you don't know who the patsy is, you're the patsy."

=======.

This is from the 1987 Chairman’s letter to the shareholders.

People usually remember the poker part of the quote but when you read the whole paragraph (which I split into two for readability), it becomes clear what he was alluding to:

People who let the market dictate their action, usually end up being the patsies in the stock market because they only know price and volume and not the underlying businesses.