The speaker begins with a small surprise: when things look simple and unadorned, he slips into "abstraction." You might expect the opposite, that complicated things would make a mind drift off. But here, a flower that just flowers and wood that just makes wood seem to free him from the effort of thinking about them. The things are so fully themselves that his mind lets go.
The last stanza makes this physical. He is "joined to a walk," or tuned, almost by accident, to the "sting / of a saw's thrust." That word "sting" is sharp and slightly surprising, giving the sound of the saw an edge. The mood is quiet, absorbed and a little tentative, shown by the opening "I think." The speaker doesn't claim to be sure. He simply notices how attention and the world can line up, without any grand purpose.