What does Heard melodies are sweet but those unheard mean?

"Heard melodies are sweet, but those unheard are sweeter" is a quote by the English poet John Keats, found in his poem "Ode on a Grecian Urn". The quote suggests that the imagination can create things that are even more beautiful than those that exist in reality. Imagination can provide pleasures and wonders sometimes beyond what one witnesses in actual life.

In the poem, Keats is addressing a Grecian urn, and commenting on the beauty and emotion it evokes. The quote refers to the music that is implied or depicted on the urn, but isn't actually heard. Keats suggests that the music that remains unheard, existing only in the imagination of the observer, might hold even greater beauty than the music that can be heard.

The quote can also be interpreted more broadly to suggest that the things we can't know or experience directly might hold more mystery, intrigue, and beauty than the things we can. It encourages us to contemplate and appreciate things that are not immediately accessible to our senses and highlights the power of imagination and introspection.