What is the meaning of [A thing of beauty is a joy forever]

The experience of beauty is blissful and lasting.

From John Keats’ epic poem, Endymion, 1818:

A thing of beauty is a joy for ever:
Its loveliness increases; it will never
Pass into nothingness; but still will keep
A bower quiet for us, and a sleep
Full of sweet dreams, and health, and quiet breathing.

Keats was writing just after the closing of the Age of Enlightenment where several British philosophers, notably Shaftesbury and Hume, had made a close study of beauty and aesthetics. Their generally accepted conclusion was that beauty was closely aligned to moral goodness and could only be fully appreciated by those of a moral and refined character. Keat’s approach was more as a practitioner than a theoretician. He responded emotionally to beauty and saw it as available to anyone who had eyes to see.