The metaphors in these introductory stanzas draw a web of connections between the natural world, poetry, and the abstract idea of beauty itself.
A "thing of beauty," the speaker says in the opening lines:
[...] will never
Pass into nothingness; but still will keep
A bower quiet for us, and a sleep
Full of sweet dreams, and health, and quiet breathing.
Beauty, in other words, builds a leafy refuge in the soul—a restful place where you can recover your strength when the tough world has tired you out. But beauty isn't just escapism; it also, the poem asserts, allows people to weave "a flowery band to bind us to the earth." That is, the bower that beauty builds helps people to stay in the world, not to get away from it. The "sweet dreams, and health, and quiet breathing" that beauty offers make people want to go on living, in spite of all the suffering they inevitably face.
The speaker presents this suffering, too, as a place. The metaphorical journey through life takes people through "unhealthy and o'er-darkened ways," or perilous paths. Notice the speaker's return to the idea of health (or the lack thereof): Keats, who trained as a doctor and who lost more than one family member to tuberculosis, had a particularly acute sense of how much it matters to be well.
Luckily, beauty offers not only a respite from such gloomy wanderings but also a guiding light and refreshment along the way. Experiences of beauty "become a cheering light / Unto our souls," the speaker says, and form "an endless fountain of immortal drink." That metaphorical cascade might even suggest the nectar of the gods, a beverage that flows eternally while offering those who taste it their own immortality. Perhaps that's where writing poetry comes into the picture. Creating a deathless "thing of beauty," after all, is one way to achieve immortality.
The poem goes on to describe the process of writing as being like a journey—a more pleasant one than the dark and rocky stumbling of the first stanza. The scenes of the story the speaker is about to tell rise up before him like "our own vallies," as green as an English hillside in spring. He'll steer the "little boat" of his words through this imagined landscape, eventually.
But first, he'll send an advance party: a "herald thought," a personified inciting idea, which he pictures preparing a path for him. By playing its "trumpet," this herald will make a green path spring up, a path that Keats can follow.
These metaphors suggest that, for Keats, writing means collaborating with a vision. The poem itself, ready to be written, appears in the imagination as a landscape; to capture its beauty, the writer must go exploring.