I have heard the phrase "here's the thing" quite a bit in informal conversation. However, recently I read it in a column by Clarence Page, whom I consider to be a good writer. I had always thought the phrase to be a filler, like "So" at the beginning of a sentence in conversation. Is it a filler? If not, what is it?

I wouldn't really consider it to be a filler. It is, of course, informal, and wouldn't normally be used in writing unless the author were trying to write in a colloquial manner.

It really doesn't add anything to the sentence, but then I know a lot of other words don't as well, like "so" to begin a sentence, or "actually" or "literally," etc.

It used to mean a summary or specific point as far as I can remember.

I suppose, in thinking about it, that is what it means. It's just an informal way of saying it, so I was surprised to read it in an op ed article.