"The Listeners" can be interpreted a number of ways, since Walter de la Mare provides the reader with no "back story" to this brief vignette in the moonlight. And what the poem teaches us is dependent upon one's interpretation. Let's look at the basic events and then discussion a few interpretations and possible teachings.
In a forest, on a beautiful moonlit night, a man on horseback approaches an ivy-covered house. The only sign of life is a bird that flies out of a turret, within, only "phantom listeners" (13). He knocks on the door three times, each time more loudly, but there is no response from within. He shouts, "‘Tell them I came, and no one answered,/ That I kept my word," (27-28). When there is no response, he turns away, mounts his horse, and rides off.
What are we to make of these characters in the poem? The "phantom listeners" (13) are clearly people who are no longer alive, either bodies or ghosts, or perhaps both. We know that the traveler is not a ghost because we are told he is "from the world of men" (16), the "one man left awake" (31). These are the "facts" we are given.
From all of this, we can infer that the traveler made a promise to the people within, one that he was long delayed in keeping, so long that they all have died. We can speculate that he made a sincere effort to keep his promise, but that he was prevented from doing so until now. One lesson we might take from the poem is that "the world of men" (16) interferes with our ability to keep our commitments, with dreadful costs to those to whom we have made those commitments. Another interpretation is that the traveler didn't care enough about his promise until just now, which teaches us, perhaps, that a promise delayed is a promise denied, again with terrible costs, to those the promise was made to, and perhaps to the person who has made the promise, who now must endure guilt. Another way of reading this is to focus on the line that tells us he is the only one "left awake" (31), with a possible implication that had the traveler been present, he, too, would have died and be one of the ghosts now, perhaps the dwellers in the house subject to a slaughter or a disease. From this interpretation, the poem might teach us something about survivor's guilt. How many people are alive because they were not at the right place at the right time? And if one has made a promise that the failure to keep has ensured one's survival, that teaches us about another kind of cost in not keeping a promise. Would this man have been able to save these dwellers? Is he alive because he was not there? The poem does not provide us with any answers, but the text supports all of these possible interpretations.
Finally, the poem presents a strong contrast between the silent beauty of nature and the loud brutality of "the world of man" (16). The only noise in the poem is the knocking of the traveler, louder and louder, and his shouted words. We are given lovely, quiet images of "the forest's ferny floor" (4) and the "leaf-fringed sill" (8). Everything in the scene is silent, harmonious, and beautiful, with the traveler as a clamorous disruption upon it. The house, with its dead or its ghosts within, is slowly being absorbed back into nature, so perhaps we can take away from this poem the idea that there is beauty in nature taking its course and that the "word of man" (16) is an ugly intrusion upon it.