I have heard about this. All the obviously good ideas have already been implemented, and others find your not-so-obvious ideas hard to understand. While this makes perfect sense, there is a chance that the people who get to know of your secret idea will understand it very quickly, and proceed to work on it.

The other advise about hiding ideas, which has nothing to do with this quote, is that its the execution of the idea which matters, not the idea itself, which is also very good advise. But again, someone else who steals your idea may be in a position to execute it better than you (more funding, more resources)