The poem is dedicated "To Christ our Lord." The reader immediately knows, then, that the poem will explore some aspect of Christianity and the relationship between humanity and God. (The dedication also informs the target of the phrase "O my chevalier" in line 11, which could relate to the windhover, to Jesus/God, or, quite plausibly, to both.)
A windhover is a bird of prey more commonly known as a kestrel, a type of falcon. As the name "windhover" suggests, the kestrel is a skilled hunter able to ride air currents with ease and dexterity, waiting for the perfect moment to swoop down and catch its prey. Indeed, this is ability to seemingly pause mid-flight forms an important focus point for the speaker's sense of wonder (and, in turn, how this impressive bird speaks to God's presence in the world).
It's worth noting the precise choice of verb the speaker uses to describe the encounter with the kestrel. The speaker doesn't say I saw "this morning morning's minion" (by which the speaker means the kestrel itself). Instead, the speaker "caught" the kestrel—which, of course, isn't meant literally. The speaker catches the rare beauty of the bird in its element, a brief moment in time that offers the speaker a powerful reminder of God's majesty. The word "caught" also relates to the kestrel, which is hovering in the air looking to catch a meal. This subtly suggests a kind of interconnectedness between different parts of nature (in this case, the speaker and the kestrel) before the poem has even really gotten started.
The poem is typical of Hopkins's work of this period in its rich and vibrant use of devices like alliteration, consonance, and assonance—all of which appear in these first two lines. Hopkins deliberately delays specifying the subject of the poem (that is, the bird) until near the end of line 2, instead offering three distinct ways of describing the bird metaphorically—and each of these is thick with sound patterning.
The kestrel is "this morning morning's minion," the first "morning" relating to the timing of the speaker's experience, and "morning's minion" suggesting that the bird is the favorite and/or servant of the morning time more generally. Both definitions of "minion" are useful, but the idea of being a servant is especially appropriate in a poem that discusses the relationship between humanity, nature, and God. The alliteration, assonance, and consonance (highlighted above) in the phrase suggest perfection and a kind of precision—a quality that the speaker perceives in the aerial abilities of the bird.
After line 1's caesura (in the form of a comma before "king"), the bird is described as the prince ("dauphin"—not meaning dolphin!) of the "kingdom of daylight"—with both kingdom and light suggesting a kind of divinity (as in, the kingdom of heaven). The third description of the bird ends up being clearest:
[king]dom of daylight's dauphin, dapple-dawn-drawn Falcon,
The sheer quantity of /d/ sounds here suggests an abundance of beauty, with the delay of "Falcon" (the family of birds to which the kestrel belongs) making the reader experience the poem's beautiful sounds in a kind of climactic build-up. The bird is "dapple[d]" because it has a spotty pattern on its feathers. (Hopkins explores the relationship between God's majesty and "dappled things" in a poem written around the same time, "Pied Beauty.")