Each mortal thing does one thing and the same: Deals out that being indoors each one dwells; (5-6)
It's interesting to note that the speaker extends this idea of selfhood in action to "Each mortal thing." Even plants and animals, by this measure, would be capable of acting in accordance with their "being indoors." Actually, maybe it's a lot easier for them to do that. Aren't human beings the only creatures that act against their true identities, their inner natures?
Selves — goes itself; myself it speaks and spells, Crying Whát I dó is me: for that I came. (7-8)
Finding yourself is a pretty big deal, but expressing it is even more important. These lines tell us that the whole reason we're on this planet in the first place is to match our self to our actions. By doing that, the actions we perform—just like the flight of a kingfisher—communicates our true identity to the rest of the world.
I say móre: the just man justices; (9)
Take a page out of the just man's book. He acts according to his inner makeup. His action is so closely tied to his inner identity that it's transformed on a linguistic level. Hopkins has to make up a new word to get that point across: "justices."