Of course, the other question people ask (after - or sometimes before - they've found out what name you use) is what your book is about.
I find this question almost impossible to answer. Synopses are hugely difficult; the kind of mini-synopsis people are asking for is worse. Even trying to give a mini-synopsis of, say, Peter Rabbit would be bad enough ("well, there's this family of rabbits, and the baby rabbits are told not to go into someone's garden. No. Wait a minute. Their mother is going shopping, so they're on their own. She's a widow, because Mr. McGregor - he's the one whose garden they're not supposed to be going into - ate her husband" etc etc). To express the meaning/intent/plot of a WIP (or finished novel) is worse.
How do you sum up the plot of your WIP/finished novel? In a couple of sentences? Is there a knack? I'd love to know.