The speaker is wandering through a dream, moving from strange worlds and unfamiliar people into the hidden corners of their own mind. At first it feels like curious exploration: "discovering more, delving deeper." But there is an undercurrent of longing too, since the speaker is searching for "one thing" they can't find, and the poem never says what it is.
The middle section gives us lovely images, such as faraway towers and beautiful rivers, but happiness appears "only through mirrors." That suggests it is a reflection rather than something real or within reach. Then the mood turns dark when corpses start washing up on beaches. The final lines pull back with a quiet jolt: the speaker is "still only dreaming." The effect is unsettling and a little sad, as if even the escape of a dream can't be trusted to stay beautiful, and waking or not waking offers no clear comfort.