Lines 12–14 read as follows:
Tossing their heads in sprightly dance.
The waves beside them danced; but they
Outdid the sparkling waves in glee:
Here, the poet is referring to the effect that the daffodils had upon him on that wonderful day when the golden host of flowers fluttered before him, putting an end to his loss of inspiration and his lonely wanderings. Although there were sparkling waves next to them, the daffodils seemed much more alive than the water as they swayed in time to the breeze, almost as if they were dancing. The mood of these three lines is one of immense joy. The dancing of the daffodils perfectly matches the emotions of the poet. He remembers just how inspired he was upon seeing the daffodils and how much they fired his poetic imagination. That's what Wordsworth means when he refers to the "wealth the show to me had brought."
As well as joy, there is gratitude in these lines. The daffodils rekindled the poet's artistic imagination, and if he ever finds himself short of inspiration, all he has to do is recall the memory of those lovely daffodils; once more he is sent into transports of delight:
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.