Like the previous editor noted, the poem is entirely allegorical and whatever happens without its metaphorical meaning is not much of what you can call "action."
With its symbolic meaning added, however, what is happening is that the man is contemplating life in a rare opportunity to "slow down" and "smell the roses" as his horse and he stopped by someones woods, who looked quite lovely during the winter snow. He feels as if even though it is nice to admire that which we want, we must continue to work in our own life's game plan. We must continue to be what we are meant to become and its OK to rest at times, but we need to keep moving.
If all you are talking about is what is literally happening, then there is not much to summarize given how short the poem is.
In the first stanza, not much is happening. All we know is that the speaker is stopping in the woods to watch the snow fall. He knows who the woods belong to, but the man lives in the village and won't see him.
Nothing actually happens in the second stanza. In the third, the pony shakes himself. This causes the bells on his harness to jingle.
Nothing actually happens in the fourth stanza, but it is implied that they drive off.
In this poem it is explaining the journey in life itself. You have to face the cold world and when you want to stop and look and the precious things in life you can't because someone is pushing you or pressuring you in the opposite direction. The last sentence could represent death instead of "Sleep" that there are so many things to do in life before you die and if you give up then you let down people that counted on you the most.