What has the speaker said yes to by continuing his passage instead of remaining in the woods in "Stopping by Woods on a Snowy Evening"? In other words, how has he resolved his conflict?
The speaker's conflict was whether to hang out in the woods and watch the snow fall (in other words, to do something fun that he wants to do) or whether to get going and head back to the real world where he has things he needs to do (obligations and duties).
It would appear (from what the last stanza says) that the speaker has resolved the conflict in favor of obligation. He seems to have said "yes" to the call of duty. He has remembered that he has "promises to keep."
From my point of view, the way I personally absorbed the poem was to think that the main character said "yes" to accepting his situation as someone who cannot just stop and rest, but someone who has lots of responsibilities ahead and maybe sometime might get the opportunity to enjoy his own moment alone in his own woods.
Its, I think, open to interpretation.