What is meant by An endless fountain of immortal drink Pouring unto us from the heavens brink in John Keats Poem A Thing Beauty?

The phrase "An endless fountain of immortal drink/Pouring unto us from the heavens brink" appears in John Keats's poem "Ode on a Grecian Urn." In this context, it refers to the beauty and inspiration that the urn provides to those who see it. The urn is described as a source of eternal beauty that can uplift and rejuvenate the human spirit, hence it is compared to an endless fountain that pours an immortal drink to all.