I think the poem will be about a man stopping in the woods on a snowy evening.
Why are we stopping?
The person that owns the woods lives in the village, the horse thinks its odd to stop without a farmhouse near. He asks why they are stopping by shaking his bells, the woods are nice but He has promises to keep.
We have to enjoy it while it lasts
bells shake, downy flake, woods fill up with snow
Happy to stop but he knows he must persevere on.
But I have promises to keep. The poem is about stopping to enjoy life before you have to move on.