What is the central idea of the poem "To the Cuckoo"?

"To the Cuckoo" presents several ideas which recur elsewhere in William Wordsworth's poetic canon. As one of the founding Romantic poets, Wordsworth was strongly interested in the idea of nature as having the power to fill us with emotion and, moreover, to sustain us after we are removed from the natural setting. Wordsworth also felt that nature has the strongest effect upon us when we are children and have not yet lost our sense of wonder. In this poem, the cuckoo bird is used to symbolize these ideas. Specifically, Wordsworth notes that the sight of the cuckoo reminds him of his boyhood; the cuckoo was, in those days, a symbol of "hope" to the young Wordsworth, and seeing the cuckoo now helps the poet to return again to the "golden days" of his youth. The cuckoo seems to stimulate those powerful feelings once evoked in him by nature; where once the cuckoo seemed to make the earth feel a "faery" world, this is something that can be ground out of a person by the struggle toward adulthood. But when the cuckoo, like spring, returns, it can remind us of how free and filled with joy we once felt when faced with simple beauty.

William Wordsworth was a leading figure of the Romantic movement in English literature. Romantic in this case does not mean that he wrote only about romantic love, but that he derived inspiration from emotion and from the contemplation of nature.
In "To the Cuckoo," Wordsworth addresses the cuckoo, a bird known for its melodious call, which heralds the arrival of spring. Hearing the bird reminds the poet of his boyhood, in which he would hear the cuckoo and pursue it through the landscape. However, the bird is gray and inconspicuous and therefore seldom seen.
To Wordsworth, childhood is a time in which the human soul gradually loses the memory of its divine origins. (In "Ode: Intimations of Immortality from Recollections of Early Childhood," lines 64–65, Wordsworth writes, "trailing clouds of glory do we come / From God, who is our home.") By reminding him of his boyhood, the cuckoo reminds the poet also of the divine origin which he and the bird, as part of God's creation, share. This enables him to see the earth as

An unsubstantial, faery place;
That is fit home for Thee!

Wordsworth would have been aware that the song of the cuckoo and its role as a sign of spring had been celebrated in many works, including one of the earliest poems in English, "Sumer is ycomen in," dated to the twelfth century.