It's something that each of us writers needs to have for projects that are complete or near complete. If it is a work in process, then you might not even know yet, and it's probably too early to talk about. But for anything that's worth a query letter, you need to have a few sentence description about the theme of the work.
That's the key, the theme. It's not the complete plot, all the characters, the entire back story. It's just a quick description of the theme set in such a way that you peak the listener's interest and they want to know more.
For instance you wouldn't want to say:
It's about this guy named Frodo who lives in the shire, and he's got this friend Sam Gamgee and they leave to go find Mt Doom cause they need to ..... Well he has this ring that ....You've already gone a few sentences, I haven't heard what the book is about, and by now I'm probably wondering what I am going to have for lunch.
Here's my latest.
It's a novel about a blinded woman's struggles to get away from her controlling husband even though he just restored her sight.Now I didn't say anything about the fact that the woman's name was Ellen, her husband is William, she has a friend name Jesse who's a biker chick, that Ellen decides to get her own bike and go riding with the girls. But aren't you wondering why she went blind, how her husband controls her, and why she's still trying to get away even though she can see again? The story is quite complex, but when you boil it down to its essence, the theme is a blind woman who can suddenly see again. That's what you need for your story. A quick description to set the context for your listener to ask more questions.
They may ask the character's names. They may ask where the story takes place. They may ask a number of things about the story, the point is, now you have their attention and your answers are helping to put the story in a framework that they can quickly understand.
So let's go back to Lord of the Rings. I haven't worked on it very hard, but here's a quick attempt.
It's a trilogy about a member of a diminutive race who overcomes tremendous odds to destroy a magic ring whose power threatens the known world.Work on your elevator pitch, and be ready to answer when someone asks. So what are you working on?
No really, I want to know. What are you working on?