The one sentence pitch is the essence or heart of a book, the line that tells people about your book and makes it sound awesome.
Example: a one sentence pitch for Eat Pray Love, might be:
“A recently divorced woman (OPENING CONFLICT) travels to Italy (QUEST) for pleasure, India for spirituality, and Bali for balance (INNER OBSTACLES), but she finds love instead. (FLAVOR)”
For my new novel, my one sentence pitch boils down to:
When an MI6 spy is assigned to unravel the largest bank robbery in history, he discovers that it is not about the money.