[Showing is...] Letting the audience put together the clues; presenting simply a sensory experience through the lenses of a character’s viewpoint. Passing no judgment, but allowing the characters to speak for themselves.
Down in the comments, he adds this:
Telling can be very useful. Think about when you tell a story to a friend over coffee. "So, my brother, right? He's totally paranoid. Get this--yesterday..." And instead of showing me paranoid, you told it to me, but that's the only way to make the following story funny or interesting or whatever. Telling is usually reflective of your point of view character.
So he's using "Tell" to include viewpoint attitude or tone, rather than just being a level of detail or summary in the writing.