It relates most directly to Francis. He has lived most of his life as a coward. He might be rich, but he never knew bravado and fearlessness like Wilson. This is one reason it is revealed that Margot had cheated on him in the past. After fleeing from the lion and Margot's subsequent affair with Wilson, he begins to hate Wilson so much that he conquers his fear. When they are hunting the buffalo the following day, Francis is totally transformed. Margot recognizes this too. So does Wilson. However, Margot kills him when they are trying to shoot the final wounded buffalo. Ultimateyl, in death Francis is able to die bravely and without fear.
For Margot the idea of not being able to dominate and manipulate her husband is frightening. Wilson speculates that Francis will leave her when they return to England. It is never clear whether Francis would have done so or not, but the fear in Margot is unmistakable. She is pale and totally changed from the day before when she was humiliating Francis after his debacle with the lion. Ultimately, she puts an end to his new life by shooting him. Whether this was intentional or personal is debatable.
Wilson is envious of Francis, for he doesn't have the money Francis does to allow him to simply hunt, which is what he loves most. But he at least can live life without fear.