I wandered lonely as a cloudThat 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 lineAlong 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.