A walker heading through the woods at evening comes across something unexpected: a ship. Of course, there's no real vessel among the trees. "Timber" is the clue. It means standing trees, but it also means the wood a ship is built from, and the speaker lets the word do both jobs. The forest becomes a hull, and the walker becomes its owner: "a ship of my own."
The ship is "full of night," as if darkness were cargo, loaded on and carried quietly. And it is "quiet and old," which suits both the woods and the hour. The mood is hushed and calm, with a little wonder in it. The speaker doesn't sound frightened by the dark. They sound pleased, even a bit possessive, as though the evening had handed them something private.
Nothing much happens, and that's the point. A simple walk turns into a small voyage of the imagination, and the poem leaves us there, drifting in the dusk.