A speaker watches an old hen pecking around a dooryard and, in that small, ordinary scene, finds a mirror for his own situation. The hen is aging, slow, and busy with the ground in front of her. The speaker, too, is aware of time running short. "Before winter happens" suggests both the coming season and the later part of a life, and the title, "Mortal," makes that second meaning hard to miss.
What the speaker feels is a quiet, stubborn wish to live. He says he has "a mind to" do it, a plain, almost folksy phrase that keeps the feeling humble rather than dramatic. The ending adds a wry twist. God is compared to "a good wit," someone who plays a clever joke, and the joke is that we are made to feel at home in a world we will have to leave. The tone isn't bitter, though. It's gentle and amused, with a touch of sadness. The comfort of the dooryard may be a trick, but the speaker seems willing to accept it.