The speaker begins by admiring the fast, restless animals of the woods: an ermine that vanishes into snow, a hummingbird darting through the trees, a shrew that moves like a missile. There's even a wink at the old typing phrase about the quick brown fox. Calling them "fast breeder reactors" makes their energy sound almost dangerous, as if they can't stop burning.
Then the poem turns on itself. The speaker is a "speed bump on a log," a lazy dog, and the tone is half rueful, half playful. The final lines flip the complaint into a quiet defense: if all those creatures make a maze, someone has to sit at its center. Being still stops looking like a failure and starts to look like a role, maybe even a kind of balance. The mood is light and humorous, but it settles into calm, gentle self-acceptance.