Step 1: This question is about the content of the stories told by the title character in Walter de la Mare's poem "Martha."
Step 2: The poem describes Martha telling stories to a rapt audience, creating a magical and enchanting atmosphere.
Step 3: The poem specifies that her stories are filled with "fairies and gnomes" and other magical elements. These are all examples of 'supernatural beings'. Her storytelling transports her listeners to a world of fantasy and wonder, away from the ordinary.