The speaker in Stopping by Woods on a Snowy Evening finds himself attracted to the scene of the snow falling in the woods and stops to observe the "woods fill up with snow" while listening to the wind lightly blowing in the background.
While it's nice in life to appreciate nature and to take time to be aware of the surroundings, the speaker has commitments and obligations that need to be honored. S/he was traveling to some destination at the start of the poem and has to stop the horse in order to observe the snow. Regardless of the attraction of the "lovely, dark and deep" woods (symbolic of the dark and deep rest of death), the speaker has to move on because s/he has "promises to keep, And miles to go before I sleep." (die)
However reluctantly, the speaker chooses to continue with the tasks of life at this point in time rather than journeying into the snowy woods of death.