Just been looking for a summary of this poem on the Internet and there doesn't seem to be one. Very briefly, I would say the essence of the poem is that it celebrates the imagination as being almost better than reality; because reality fades or becomes boring, while the imagined world never does, and in the imagination (or Fancy) you can be anywhere, or with anyone you like - see and do whatever you like. (I think he is supposed to have written it sitting by the fire in December 1818, but you can see a lot of the poem is about light, sunshine etc - proving that the imagination sets you free from your circumstances. Hope this helps.
Unlike P.B. Shelley and William Wordsworth, John Keats is the only poet who separates present from past and future. Due to this quality, Keats is famous as an "Escapist". He preaches that in order to be happy one should escape from reality and take a refuge in "Fancy". (As a famous saying is: Only brave people can live in reality.)
Keats uses his ideology of escapism in his "Ode to Fancy" as its main idea. When reality is unbearable, a person should get rid of it through imagination. He uses Greek myths as reference and inspires the readers to use their fancy to up-end the boring influence of reality.
Through imagination one can go anywhere, do anything and enjoy the things that are perhaps impossible in reality. A person can even fly with imagination, as Keats addresses it as "Winged Fancy".
Imagination brings colors and provides variety in human life. As variety is the spice of life, imagination is very important to live a happy life whenever reality is unfavorable.
So, "Ode to Fancy" by "John Keats" is all about imagination and its importance in human life.