I get this question a fair amount. You mention you're writing/editing/plotting xyz and people ask "What's it about?" They mean to get you involved in a conversation. They mean to be encouraging and show interest and support in your passion. But there are a few reasons this doesn't really work for me.
3. It's Too Big
I know once I get something ready for submission I'll need to be able to hook people in under 250 words. I know I'll need to be able to say what it's about in two or three paragraphs. But when I'm writing, the truth is, there's just too much. I mostly write long fiction. You ask me about a short story and I can probably say what it is. You ask me about a novel in progress... not so much. My first question will be "Do you want that in terms of plot or theme?" and my second will be "How much time you got?"
2. The More I Talk, The Less I Write
This is a pretty big one. I've gotten out of the habit of sharing my work and talking about it while it's in progress (unless it's to share snippets with my writing group), because the more I talk about it the less I actually write it. I know from experience that once a piece of it is out in public, I grow progressively less likely to write the rest of it. So this question is actually counterproductive.
1. I Don't Know
When I started Voice it was called Diary of a Monster and it was about the things we do that hurt the people we love. When I finished the first draft I renamed it Price Tag and it was about the value of a life and the price worth paying for what we want. Now it's called Voice and it's about adapting to loss and grief. Inked has gone through a number of transformations as well, but in that case it's more to do with the characters and flow and less to do with the theme. Once again, eventually I'll need to know, but until it's done I really, really don't.
These are of course my personal thoughts. Does anyone honestly enjoy the question? Do you have other problems with this than I do?