If you can't explain it, it's probably not such a great idea as you think it is.

I think that depends very much upon the audience. There are quite a few great ideas out there that are difficult to explain: try explaining the theory of general relativity to your mother, for example.

"I can't explain it" does not depend on the audience, it depends on your ability to understand and explain it.

This is why I didn't say "I can't explain it to two-year-olds".

My mother is technical and well educated. She probably understands General Relativity better than I do.