"Mowing" begins with a simple setting and concrete imagery—but quickly shifts into a more abstract, metaphorical realm.
The title announces that the poem is about the ordinary chore of "Mowing." In lines 1-2, the speaker introduces himself as someone who once did this chore regularly. Since the poem dates from the early 1900s—before the widespread use of modern, gas-powered mowers—the speaker did the job with an old-fashioned hand tool: a "scythe." (Non-motorized push mowers, steam-powered mowers, etc. were introduced in the mid-1800s, but scythes were still in common use for many decades after—for example, to clear tall grass in advance of these other tools. In some rural parts of the world, scythes are still used today.) Ultimately, the poem explains that the speaker cut the grass to make "hay"—livestock feed—implying that he's a farmer or farm worker.
The speaker used a "long scythe": one whose curved cutting blade is attached to a long handle, meant to be swung with two hands. When the speaker mowed, he worked in a field beside a "wood," and the swish of his scythe blade made the only sound within earshot: "There was never a sound beside the wood but one, / And that was my long scythe whispering to the ground." Notice how these lines are full of soft /s/ and /w/ consonance, mimicking the "whispering" of the scythe itself:
There was never a sound beside the wood but one,
And that was my long scythe whispering to the ground.
This type of consonance and alliteration will continue throughout "Mowing," reinforcing the poem's imagery through the sound of its lines.
The speaker then poses a playful rhetorical question about the scythe:
What was it it whispered? I knew not well myself;
In other words, if the blade was whispering through the grass, what was it saying? The speaker confesses that he doesn't know the answer "[him]self." And, of course, the question is impossible to answer on the literal level: the scythe wasn't actually saying anything to the ground. But the speaker takes this playful conceit and runs with it, personifying the scythe and imagining what it would be saying if it could speak. As the poem goes on, this conceit becomes a way of discussing the larger meaning of the speaker's work—as a farmer and perhaps as a writer, too.