For oft, when on my couch I lie In vacant or in pensive mood,
Now the speaker explains why the daffodils were such a great gift to him. He moves suddenly into the future, back from the lake and the windy day. He’s describing a habitual action, something he does often.
First, he sets the scene: he often sits on his couch, kind of feeling blah about life, with no great thoughts and sights. Sometimes his mind is empty and "vacant," like a bored teenager sitting on the sofa after school and trying to decide what to do. At other times he feels "pensive," which means he thinks kind-of-sad thoughts. You can’t be both "vacant" and "pensive" because one means "not thinking," and the other means "thinking while feeling blue." But he groups the two experiences together because both are vaguely unpleasant and dissatisfying.
They flash upon that inward eye Which is the bliss of solitude;
So, often when our speaker gets in these downer moods, the image of the daffodils "flashes" through his mind.
The "inward eye" expresses what Wordsworth felt to be a deeper, truer spiritual vision. A person cannot share his or her own spiritual vision completely with others, and so it is a form of "solitude." But its truth and beauty make it "blissful."
Why does the speaker think of daffodils in exactly these moments? Maybe it's because the contrast between their joy and his unhappiness is so striking. Nonetheless, the vision is spontaneous, like a crack of lightning.
And then my heart with pleasure fills, And dances with the Daffodils
When the memory of the flowers and the lake flashes into his head, he feels happy again. It’s almost like the same experience he had while "wandering" through nature at the beginning of the poem, when the real daffodils pushed the loneliness out of his head.
The memory of the daffodils is as good as the real thing.
His heart is set to dancing, just like the flowers. He dances along "with" them – they are his cheerful companions once again.