A summary, as the one sentence requirement emphasizes, is a quick highlighting of the most important points of a story, with the less important details left out. A summary gets straight to the point. One possible one sentence summary of "The Lottery" could be as follows:

In Shirley Jackson's "The Lottery," residents of small New England village have an annual lottery in which one member of the village is chosen as a human sacrifice to be stoned to death because of the superstitious belief that this will insure a good harvest.

After getting to the heart of the story in a sentence similar to that one, you would then be free to add details about the plot or talk about the point the story is trying to make.

"The Lottery" is a short story written by Shirley Jackson.  The story is set in a farming village of "about three hundred people" and takes place on a beautiful summer day.  When all of the able-bodied citizens meet in town, a lottery, or drawing, is held; the "reward" for being the winner of the lottery is not disclosed until the end of the story, although a significant amount of foreshadowing suggest an unpleasant outcome.  Jackson also uses a great deal of symbolism in her story and provides a unexpected and ironic ending that aids in conveying the idea of the story's theme to the reader.

A one-sentence summary of "The Lottery" might go as follows:

The "winner" of a small farming village's annual lottery is sacrificed for the good of the crops.

or

Superstition leads a small farming village to conduct a yearly lottery in order to make a human sacrifice.