A speaker sits alone and is perfectly pleased about it. The opening line is a joke with a grain of truth: being "king of the yard" makes solitude sound like a small private kingdom, grand and slightly silly at once. Nobody else is there to challenge the title, so the speaker wins by default. The comparison to a fish adds to the sense of ease. A fish doesn't long for anything outside its water, and the speaker seems equally at home in their own element.
The second stanza turns to reading or writing, "the company of letters." Here the speaker says they don't find people "so wanting," which is a careful, understated phrase. It isn't a harsh rejection of others, just a quiet admission that books and words fill the space well enough. The mood is calm, wry, and gently self-satisfied. There's a faint hint that the speaker may be protesting a little, but the poem mostly reads as a small, contented moment of being left in peace.