Atmosphere is the way an author describes specific places to make readers feel a particular emotion. Atmosphere differs slightly from mood in that it is broader and can apply to a location, while mood applies more to a person or people's emotions. A literary piece's atmosphere can help readers determine what the piece's genre is.
"Ten minutes of determined effort brought another sound to his ears—the most welcome he had ever heard—the muttering and growling of the sea breaking on a rocky shore. He was almost on the rocks before he saw them; on a night less calm he would have been shattered against them. With his remaining strength he dragged himself from the swirling waters. Jagged crags appeared to jut up into the opaqueness; he forced himself upward, hand over hand. Gasping, his hands raw, he reached a flat place at the top. Dense jungle came down to the very edge of the cliffs. What perils that tangle of trees and underbrush might hold for him did not concern Rainsford just then."
This portion of "The Most Dangerous Game" by Richard Connell establishes the atmosphere of the island.