The poem begins with a question the speaker can't quite answer: what did I hear in the park? The answer, "many things," is deliberately loose, as if the ear took in more than it could sort. Then the focus narrows to "calico flags," patchy, homely pieces of cloth that seem to fit "their meaning" without ever spelling it out. The speaker goes to them "to understand us," which suggests a wish to learn something about people, not just about the park.
What they find is not an explanation. The ending offers "few words but the water / high and singing." Understanding comes through sound rather than language, and the water seems to say more than speech could.
The mood is quiet, curious, and a little humbled. The title, "Known unknowns," fits: the speaker senses meaning is present but doesn't claim to have pinned it down. The poem seems content to listen, and it leaves the reader doing the same.