The above-quoted lines are taken from William Wordsworth’s poem “I Wandered Lonely as a Cloud” or “The Daffodils”, as it is popularly known.
 
Here the poet describes the beautiful scene which he had seen while traveling in the Lake District of England. The daffodils flowers were fluttering and dancing in the breeze beside a lake and underneath the trees. 
 
Now, these lines are especially important in explaining how beautiful the nature was there. It was almost a heavenly sight with all the elements present there at once — the lake, the trees, the flowers and the mild breeze. Moreover, the “fluttering and dancing” of the daffodils convey that the poet was very much charmed at the sight of those flowers, ending up describing those as if they were human beings.