The poem starts with a small, odd question: how many times does a fly beat its wings? The speaker doesn't answer it. Instead, the answer is something "the leaves tally," as if the natural world keeps its own books, counting what we never could. The title, "Accounting practice," plays on that idea, since nature seems to do the bookkeeping without any effort.
In the second stanza the speaker places "us" in a nest, a cozy, sheltered spot where we take "pleasure in everything." The one exception is the moon, called a "pretty clock." That phrase suggests the moon is a reminder of passing time, something lovely that still counts down our days. The poem seems to say that while the leaves tally quietly, we would rather not be reminded of how much time we have.
The mood is gentle and curious, with a slight shadow of mortality under the contentment.