Adopt the method of integrating feelings into the scenery. The third sentence says that the light in the cool porch reflects the bright moon on the anvil, creating a quiet (or cool) atmosphere. Taking care of the last sentence, writing a letter from home adds endless troubles and expresses the poet's strong homesickness.
Poetry creates an image of a traveler who has been homesick for a long time, but is aroused by receiving a letter from home.