The title sounds to us like it would work nicely as a title for a painting, and we can easily imagine (even before reading the poem) gazing at a framed oil painting in a carefully lit museum that shows a figure paused in a dark and snowy landscape. The "-ing" ending to the word "stopping" (making it, in fancy grammatical terms, a gerund) gives us the sense of the immediate present, as though we are just now watching our speaker stop to take a gander at the woods. This "-ing" ending also makes us feel as if things are in motion, and as if the speaker is in the middle of a journey or task.
If we were to award a gold medal to the word in this title with the juiciest meaning, the word "by" would be the lucky winner. When we think of woods, we imagine being in them, surrounded by trees. However, our speaker is not in the midst of a great forest: he's actually just next to the woods and staring at the trees. Staring at woods strikes us as just a wee bit strange. We can imagine hanging out with trees, because trees are cool. But our speaker is on the periphery of the woods; he's separate from them. Before we begin the poem, Frost makes us aware of the fact that the speaker is not inside the woods, but is rather beside them. And that strikes us as just plain interesting.