I wandered lonely as a cloud That floats on high o'er vales and hills, (lines 1-2)
If you’re a connoisseur (a big follower) of Wordsworth’s nature poetry (and who isn’t?), you’d probably guess that he’s talking about the famous Lake District in England. Why? Because he’s always talking about the Lake District, his favorite wilderness region this side of Sussex. At the same time as he identifies himself with a part of the surroundings, the clouds, we imagine that he is also "wandering" through the area.
They stretched in never-ending line Along the margin of a bay: (lines 9-10)
Wordsworth compares the "line" of dandelions stretching from one end of the bay to the other to a band of stars in the Milky Way stretching from one end of the sky to another. He takes an image from nature on earth and makes it seem heavenly, or at least astrological.
Ten thousand saw I at a glance, Tossing their heads in sprightly dance. (lines 11-12)
That’s a lot of daffodils. That would give you, like, 100 flowers for every day of the spring.
I gazed--and gazed--but little thought What wealth the show to me had brought: (lines 17-18)
The speaker watches the flowers for a long time. He must be hypnotized. Interestingly, although he appreciates the scene at the time, he needs time to process and reflect on nature in order for its full value to become clear to him. It’s not a sudden revelation or epiphany.
For oft, when on my couch I lie In vacant or in pensive mood, They flash upon that inward eye (lines 19-21)
Nature by itself does not create the feeling of deep joy and peace. It merely provides the raw materials. Nature must be worked on by the active imagination, the "inward eye." Note that Wordsworth does not claim that people can be transformed by nature all the time. They have day-to-day concerns that keep them indoors much of the time. But the daffodils are like that screen saver you have on your computer of some warm sandy beach in the Bahamas, reminding you that good stuff is out there in the natural world.