Three short lines give us one strange, vivid picture: layers of glass caught in a thermal, the current of warm air that lifts birds and gliders, and spinning upward. It's hard to say exactly what the glass is. It might be actual panes, or it might be a way of seeing something else, like light flashing off wings or a shimmer in the sky. The poem doesn't explain, and that openness is part of its pull.
The speaker's feeling comes in the final line, a question: "what can stop them now?" It sounds half amazed and half uneasy. The glass is fragile, yet it moves with a force that seems beyond anyone's control. There's delight in the spinning, but also a small hint of danger, since glass that rises can fall or shatter.
The mood is bright, suspended and a little tense. Because the poem is so short, it leaves you hanging in mid-air, just as the glass is, with no answer to the question.