What does which is the bliss of solitude in the poem Daffodils mean?

For oft, when on my couch I lie

In vacant or in pensive mood,

They flash upon that inward eye

Which is the bliss of solitude;

And then my heart with pleasure fills,

And dances with the daffodils.

"Bliss of solitude" means that the poet is happy to be alone.

The stanza is commenting that when he is alone, and not feeling much

of anything, he remembers the daffodils and they make him feel happy.