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;
Or like stout Cortez when with eagle eyes
He stared at the Pacific—and all his men
Looked at each other with a wild surmise—
Silent, upon a peak in Darien.

 Darien is in Central America.  The contrast between the old world and the new is a metaphor for the Keats who was a ‘reader’, as he himself puts it, and the Keats who ventures into the adventurous world of a creative poet. The idea of a ‘peak’ is also apt; implying spiritual heights can be reached through writing. Or else he can tumble and fall. But both are what he must venture. 
 The word ‘silent’ is ironic. If Keats is using an analogy to compare himself to an adventurer and explorer in order to describe his creative forays, why should he be ‘silent’? We can speculate that it is the liminal moment between the two stages of his life, the cusp  on which his mind must pause and think deeply before finding his poetic voice.