This passage describes a scene of snowy desolation.  The narrator is riding in a wagon with Ethan Frome; they reach a fork in the road by the schoolhouse, and go to the left down a lane where snow-covered hemlock branches bend inward towards over the path.  The narrator has been here before, and recognizes the saw-mill Ethan owns.  It looks "exanimate", which means spiritless; the wheel at the mill is still, and the stream that passes through it is black, with "yellow-white spume".  The sheds "sag" under the weight of mounds of snow.  The whole scene is lifeless and bare and ugly, and Ethan does not even turn to look at it as they drive by.

The narrator and Ethan ride in silence as the wagon starts to go up a hill.  The narrator, who has never been in this area before, describes "starved apple trees writhing over a hillside among outcroppings of slate".  Imagine bare, leafless trees twistedly sprouting out of the ground, which is rocky and black, and the whole scene smothered with snow so that just parts of the stones and trees are visible.  Beyond the apple orchard are endless fields; everything is white with snow, and even the sky is a light, very pale gray, so that everything is a "white (immensity) of land and sky".  One little "lonely New England farm-(house)" appears in this desolate scene, and its isolation just "make(s) the landscape (look) lonelier".

The purpose of this description is to give the reader a feeling of just how empty and isolated the area where Ethan lives is.  As you read on, you will see that the utter loneliness of existence in this place will play a huge part in determining why the characters make the decisions that shape their lives.