Whose woods these are I think I know.
His house is in the village though;
He will not see me stopping here
To watch his woods fill up with snow.
My little horse must think it queer
To stop without a farmhouse near
Between the woods and frozen lake
The darkest evening of the year.
He gives his harness bells a shake
To ask if there is some mistake.
The only other sound's the sweep
Of easy wind and downy flake.
The woods are lovely, dark and deep,
Driven by a sense of duty, or perhaps fear, the horse seeks to rouse his master from contemplation. Instinctively he feels out of place stopping here. 
 The horse is attributed with an understanding of  human emotions and ability to communicate with the speaker. Having developed a connection to his rider, he would be lost without him. The domesticated horse represents society. At first he is ironically more drawn to returning home than his human master. 
 The “harness bells” – they confine, and communicate by making sound – may be metaphoric for language, whether spoken or unspoken, which reign us in and bring us back to our social lives. However, the speaker is moving away from society in his thoughts, which are more free to contain and express his spirituality. 
 
 The horse may carry symbolic associations, suggesting a source of strength, progress, or productivity that the speaker has temporarily lost touch with.
Stopping by Woods on a Snowy Evening