An ode is a lyric poem that typically praises someone or something. It could be a famous person, a romantic interest, a shoe, or a concept like "love."
Keats was a big fan of odes. He wrote quite a few on topics ranging from melancholy to urns to birds. These are considered the Great Odes, and they are among the most famous of his works.
In this case, we are paying tribute to the pleasures of "indolence," better known to us as laziness. Nothing can beat it, says the speaker—not love, not fame, not ambition, not even the very thing the speaker seems to be skilled at: poetry.
The pleasures of a lazy summer day, or a morning spent sleeping in are no match for anything, says the speaker. Hence, we get this ode.