"Dusk" is another classic story by Saki with a twist in its ending that surprises us all. However, your question relates to the initial paragraph, which builds up a description of the scene and gives us the setting for the action that is to follow. Note the information we are given in the first two sentences of the story:

Norman Gortsby sat on a bench in the Park, with his back to a strip of bush-planted sward, fenced by the park railings, and the Row fronting him across a wide stretch of carriage drive. Hyde Park Corner, with its rattle and hoot of traffic, lay immediately to his right.

Now, "bush-planted sward" refers to a stretch of grass or turf and obviously refers to the grassy backing that Norman Gortsby sits against. Note the way in which the sounds of the setting are brought to life through the onomatopoeia of Hyde Park Corner with its "rattle and hoot of traffic." Thus the first two sentences of this story give us important descriptive details and also create the setting where the action will occur.