Words turn into produce in this metaphor, a nice riff on the whole question of the value (or the <em>productivity</em>) of literature, which has been bugging writers since, well, the beginning of time. Flour, one of the most basic of food staples, is made from wheat. In one stroke, Keats makes language as necessary as the food we eat.
the night's starr'd face, (5)
Keats personifies the night, turning it into a figure with which he can interact. Ironically, this "face" is described in more detail than the actual "fair creature" he mentions later.
Huge cloudy symbols of a high romance, (6)
Where should you look to find signs of love? Why, in the sky, of course! Keats seems to be fashioning nature into a natural extension of his own emotional register. When he's happy and in love, the clouds bear the message up in the sky.
[…] on the shore Of the wide world I stand alone (12-13)
Nature becomes one big mirror for our speaker's soul in the last lines of this poem: the wide world is the only thing huge enough to put our speaker's all-too-healthy ego into check.