Dave Bonta's short poem sets two kinds of time side by side. In the first lines we get something slow and heavy: rocks, and an oak that has sat among them for 200 years. The word "sat" is a nice choice, since it makes the tree sound patient, almost like a person settled in for the long haul. Then the last line breaks in with a quick, bright movement: a lizard flashes by.
The speaker doesn't tell us how to feel, but the poem seems to be about noticing. Judging by the dash and the sudden switch, the speaker is looking at the old tree and the stones, perhaps thinking about how long they have been there, when a flicker of life interrupts the thought. The contrast makes the oak's age feel even greater, and the lizard's brief moment feel even more vivid.
The mood is calm and quietly wondering, with a little jolt of surprise at the end.