The most important things to know relate to the main character and his opponent, Rainsford and General Zaroff. It is important to know that Rainsford is a world-famous hunter, as evidenced by his first meeting with General Zaroff when Zaroff says, "It is a very great pleasure and honor to welcome Mr. Sanger Rainsford, the celebrated hunter, to my home" and then, a few lines further, "I've read your book about hunting snow leopards in Tibet, you see." This demonstrates Rainsford's fame and skill. One should also know that Rainsford is on his way to hunt in the Amazon, as he states at the beginning of the story. Additionally, this shows Zaroff's admiration for Rainsford, and thus the incredible opportunity for a challenge he sees in Rainsford and with the prospect of hunting him.
Zaroff tells us anything we need to know about himself when he meets Rainsford.