The mood of the poem's narrator is one of longing. He stops in the midst of a busy journey to watch the snow fall. He is in the woods, on horseback, and it is, he says, the "darkest evening of the year." We as readers can imagine the beauty of the white snow falling against the dark woods.
The speaker would love to stay right where he is, watching this beautiful and still scene. However, he must move onward. He has places to be. He really has no time to linger. As he states:
The woods are lovely, dark and deep,
But I have promises to keep,
And miles to go before I sleep,
And miles to go before I sleep.
Using simple language, the poet captures one of those fleeting moments in which, no matter how busy we are, we are so struck by the beauty around us that we feel compelled to stop and appreciate it. The poem captures the longing we all have to be less harried and to experience the world's beauty more fully.