Harbor is a single sentence, and it is a blunt one. The speaker is talking to someone else, a "you," and says that of everything this person gave, only poetry was real. The rest, the poem suggests, was empty or false, whether that means promises, affection, or something else. We aren't told what went wrong, and that silence makes the line feel sharper.
What stands out is the verb "rattle." Poetry doesn't soothe the speaker or comfort them here. It shakes them down to the bones. That could be bitterness, since the speaker seems to be crediting the giver with only one honest thing. It could also be gratitude, since being shaken awake by words is a powerful experience. The poem leaves both readings open.
The mood is stark and a little unsettled. The title, Harbor, suggests safety, yet the poem offers a jolt instead. That contrast may be the point: the one thing that feels solid is also the thing that disturbs.