The speaker is looking at a painting by his wife and finding something of midnight in it. He doesn't describe the picture in detail. Instead he gives one striking image: a bed that becomes a ship, being carried north. The title, "Mindnight," swaps one letter in "midnight" to bring in the word "mind," so the painting seems to be about the dark hour and also about the inner world, the thoughts and dreams that surface when we lie awake.
The poem is very short, and that helps its mood. It feels hushed and dreamlike, a little eerie but also tender. A bed is where we sleep, and a ship is where we travel, so the two together suggest sleep as a voyage into somewhere unknown. "Taken round" implies the bed isn't steered by anyone; it is simply carried along. The speaker seems quietly absorbed, perhaps moved, by how a painting can turn something ordinary into a journey through the night.