They showed the vast expanses of the radiance of their beauty, expanded the boundaries of my desire, and departed.

Imagine catching a glimpse of an infinite horizon—suddenly, everything you knew feels too small. The poet suggests that by revealing the sheer, boundless radiance of their beauty, the beloved expanded the lover's very capacity to desire. The sweet agony is that after stretching these limits of longing to infinity, the beloved simply departed, leaving the lover stranded in a vast ocean of unquenchable yearning.