The poem imagines the world from an old swan's point of view. The first half is quiet and settled. The swan stays in bed, which is simply "the custom," and the best place to rest is where the sun once crossed the water. There's a sense of routine, of an old creature who knows what it needs and doesn't hurry. The sun has already passed, so even the light is something remembered.
Then the poem turns. We, meaning humans, "tower and boat" over the swan, and to it we look like "blind / stairs of lead." The image is heavy and a little threatening: we are big, clumsy, and unseeing, moving through the swan's space without noticing it. The speaker doesn't say outright how they feel, but the shift suggests some self-awareness, even a touch of guilt, about how intrusive people can be. The mood is calm and tender at first, then uneasy, as gentle rest gives way to a sense of our weight.