At the beginning of the poem, the speaker is feeling lonely and sad. As he walks along, he sees a large area of daffodils along the side of a lake, blowing in the breeze with bright yellow flowers reflected in the water in spite of the waves due to the wind. The sight of the flowers on the shore and their reflection cheers him greatly. To the poet, these flowers are not just pretty, but cheerful and joyous company that brings him out of his loneliness. Best of all, the poet finds that the memory of the sight of the daffodils stays with him, giving him companionship and joy when he is "in vacant or in pensive mood."
The central idea of the poem is the expression of the comfort and cheering the author finds in the beauty of observing the daffodils.