A woman and a horse are somewhere off the road, and the poem watches them with almost no fuss. Her hand "tells things" to the animal, a lovely phrase because it suggests communication without words: touch as speech. The title, "Whisperer," hints at the old idea of someone with a gift for calming horses, but the poem quickly pushes away any sense of showiness.
The third and fourth lines say it plainly: "that plain going on / no art to it but hill and haze." Whatever passes between her and the horse is simple and ongoing, not a performance. The only "art" is the landscape itself, the hills and the soft blur of haze around them.
The speaker seems to be an observer, and the tone feels admiring but restrained. The mood is calm, hushed and slightly dreamy, as if the poem itself is trying not to disturb what it describes. It leaves you with a sense of ordinary, wordless understanding.