A wonderful commenter, Suzanna from Oregon, explained it recently on "Motherlode," the parenting blog of the New York Times.
I also found the story of it recounted, by author Mark Epstein, who was there, in his book, Thoughts Without A Thinker. (I love Google.)
Basically, Ajahn Chah, a famous Buddhist monk, whom Epstein visited in Thailand with a group, showed them his drinking glass. He told them he loved that glass, it was so pretty, and when he tapped it, it made a lovely sound, but, he explained, “The glass is already broken.” He went onto explain that it was inevitable that, someday, that glass would break. There is no way that it would last forever. Maybe he would knock it over with his sleeve or maybe it would fall off its shelf, but it would shatter and be gone.
Rather than spend effort trying to prevent the inevitable, twisting himself up with worry, maybe being grumpy to other people, maybe even packing the glass away in storage where he wouldn’t be able to use it or even see it, he chose to enjoy the glass now, while he had it, and not worry that it would someday be gone.