"As Kingfishers Catch Fire" is not so much a title as it is the first four words of the poem. Still, we all know that first impressions can leave a lasting impact, and that goes double when you're talking poetry. It's worth unpacking these four little words, then, to see how it sets the stage for Hopkins's ideas in this poem.
First off, we note the "As," which tells us right away that we're dealing with a simile here. As we soon see, once we dive into this poem (not unlike a kingfisher diving into a river in search of a tasty fish), this is the first of several similes—bringing dragonflies, rocks, musical strings, and bells into the picture as well.
Still, what's so great about this kingfisher that it both starts the poem and makes it into the title. Well, we think it has something to do with the imagery of the title phrase. A kingfisher, on its own, is really just a bird (albeit a bird with a royal-sounding name). In the poem and in title, though, these birds "catch fire." That is, their coloring makes them look like tiny firebolts as they flap through the air.
This is actually a pretty important image as far as the poem is concerned. These kingfishers are doing what they're supposed to: catching fish. The speaker of the poem would take that one step further, though, and say that they're expressing their inner being through action. A kingfisher's innermost, God-given purpose is to fly and catch fish. As it embraces that purpose, it projects its inner self into action. And that, friends, is when it really starts to glow.
In that sense, these kingfishers are models of self-realization for us all. Their actions are based on what the speaker would call their "being indoors" (6), the inner essence of their existence, the thing which makes them kingfishers in the first place. In doing so, they become more than just birds. They "catch fire" and become shining examples of what we can all become, if we just do what's true to our nature.