Much have I travelled in the realms of gold
And many goodly states and kingdoms seen;
Round many western islands have I been
Which bards in fealty to Apollo hold.
Oft of one wide expanse had I been told
That deep-browed Homer ruled as his demesne;
Yet did I never breathe its pure serene
Till I heard Chapman speak out loud and bold:
Then felt I like some watcher of the skies
When a new planet swims into his ken;
Keats' metaphor — describing Homer’s poetry as the air he breathes — demonstrates the value the poet places on literature. Literature kept Keats alive, a biological necessity.  But breathing involves exhaling, so this indicates Keats saw the importance of ‘breathing out’ his own literature. 
 
 Given his longing to produce poetry valued by society, Keats took it hard when Blackwoods and other literary magazines rejected his work.  It is ironic that this line, with its elegant,  assonantly  rhymed ‘breathe’ and ‘serene’ should be so memorable.