What does the windhover represent to poet?

The opening of the sestet serves as both a further elaboration on the bird's movement and an injunction to the poet's own heart. The " beauty," "valor," and "act" (like "air," "pride," and "plume") " here buckle." "Buckle" is the verb here; it denotes either a fastening (like the buckling of a belt), a coming together of these different parts of a creature's being, or an acquiescent collapse (like the "buckling" of the knees), in which all parts subordinate themselves into some larger purpose or cause. In either case, unification takes place. At the moment of this integration, glorious fire issues forth, of the same order as the glory of Christ's life and crucifixion, though not as grand. The speaker compares the bird with Christ, "my chevalier", who is a billion times lovelier, more brute, and dangerous in his beauty. the fire or brilliance of Christ is dazzling this bird is no wonder. "No wonder", says the poet about the bird because the real wonder of the world is another supreme gift of God, his son, the Christ. 

His steps on the soil make resemblance a wound when the blood-red and golden light of the sun is cast on it. The flight of the bird reminds the speaker of Christ's crucifixion. The last stanza associatively brings together unrelated words, each telling something about Christ and his suffering and sacrifice for human beings. The description of the first stanza and the comparison of the second stanza are all forgotten when the poet deeply meditates and exalts in the sacrifice and greatness of Christ in the last three-line stanza. The red ember-like light of the morning sun in the horizon of the blue-bleak sky and he is lost in contemplation. By implication, the poem is therefore a poem of Thanksgiving to Christ. It is a hymn that is romantic in form but religious in theme. When the poet sees the beautiful bird, he is reminded of Christ and becomes thankful and appreciative of him. The poem's theme is therefore related to the poet's praise of Christ rather than being about the bird.