Through the ample open door of the peaceful country barn,
A sun-lit pasture field, with cattle and horses feeding;
And haze, and vista, and the far horizon, fading away.
Just listen to the wonderful rhythms of the first line, so nicely divided into two symmetrical halves of trochaic tetrameter. And he keeps changing it up. So the attractive sound qualities really contribute to the pastoral tranquility. It's summer, it's good, and we see an attractive, productive landscape. But the fading horizon is a touch ambiguous. A receding picture. Infinite? In any case, consider the perspective: this picture is described from inside the barn. We see out through the open barn door, which suggests a very "pictorial" frame. To me, that's one half of what the title is about. Our frame of reference is a peaceful country barn, and we are in it. Not a bad place to be on a hot summer day!
Sometimes these peaceful places may lead us to excessive pondering. I think I'd like my horizon to always be fading, but I fear it's coming closer. And sometimes the best I can manage is "it's o.k." Walt makes it easier.