What does 'perne in a gyre' mean in W.B Yeats 'Sailing to Byzantium'?

In W.B. Yeats' poem "Sailing to Byzantium," the phrase "perne in a gyre" refers to the idea of being caught in the cycle of life and experiencing continual change and transformation. It represents the restless, ever-changing nature of human existence and the desire for something eternal and timeless.