A metaphor a figure of speech which compares two things that are unrelated but do have something in common. It often states that one thing as another.
The house implies a grave where the speaker is buried. It indirectly gives some descriptions by stating that the house seemed like a "swelling of the ground", with a scarcely visible roof. This fits the description of a grave, thus the house is a metaphor for the place where the speaker is buried.
