Whitney tells Rainsford about the superstitions sailors have about the island they are passing. He asks Rainsford if he can feel the evil emanating from the island.
"An evil place can, so to speak, broadcast vibrations of evil." (page 2)
When a writer foreshadows something, he/she uses clues to suggest events that have yet to happen and to build their reader's interest and suspense. The reader has had no contact with the island yet, and yet when Rainsford falls overboard, and we hear gunshots coming from the island, we know that something evil is happening on that island. Whitney just puts the idea in our heads, and we continue to read with that expectation.
My copy of the story is from the internet, but the quote is from the beginning of the story and should be relatively close to page 2.