The title is a science word: sublimation is when a solid turns straight into gas, skipping the liquid stage. The poem opens by running water's cycle in an odd direction, water back to ice, and ice to air, as if things are being undone or thinned out until they nearly vanish.
The speaker then says "being no green man," which seems to mean they aren't some wild nature figure or expert of the woods. Instead they travel only in "our own garden," a small, familiar, shared place. The word "our" suggests a partner or household, though the poem doesn't say who.
The last lines, "a bed in our part / of night," shrink the world further, down to a bed and a slice of darkness that belongs to the two of them. The mood is quiet, intimate and a little dreamy. Things fade and change state, but the speaker seems content with a modest, close-to-home kind of wandering.