A falcon appears where people are fishing, and the poem treats it almost like a teacher. It is "simple as a master": it does without effort or fuss what the human fishers work at. That sudden contrast sets up everything that follows.
The speaker then admits something awkward: "I do not believe but must believe / in the supple art of folly." It reads like a confession. The speaker doesn't trust the idea, yet feels pulled toward it, as if some kind of wise foolishness, a willingness to be unguarded and odd, is the only way to be alive to the moment. The last lines push that further. "To see with an ear" blurs the senses, as though watching the bird requires giving up ordinary ways of perceiving. "To eat my own" is more unsettling, and it's hard to pin down. It might suggest self-consumption, or a hunter's honesty about feeding on what is yours.
The mood is spare, a little wild, and quietly uneasy, with admiration mixed with doubt.