The speaker makes a lot of the fact that he catches sight of the windhover in the morning. In case you're not a morning person, let us tell you something about it: the early morning light gives everything a kind of glow as the sun is coming up. Most of the world is still quiet, so if you're out early enough, you might catch sight of something special—like the windhover—and you'll have it all to yourself.
Line 1: Lots going on here. The speaker repeats the word "morning" in a way that might make the reader do a double-take. The first time he says it, he's saying when the poem takes place: "this morning." The second time, it's hooked up with the word "minion" (a devoted servant)—the thing he saw "this morning" is the devoted servant of the "morning." Phew! Calling a bird a minion is a metaphor, since he's not literally a servant. The poetic repetition of the word "morning" could be seen as an example of anaphora, and the repeated M sounds are alliterative. The repeated M sounds make you almost hum across those words as you read them out loud—maybe the idea is to show how effortless the bird's hovering is. He just hums along.
Line 2: More alliteration in this line—"dapple-dawn-drawn" repeats the D sound. But compared to the flowing, humming M sounds of the first line, the repeated D sounds seem a lot more staccato, or disjointed. The words dawn and drawn rhyme, but since they appear within the same line, that's called an internal rhyme.