In "The Road Not Taken," what does the "yellow wood" symbolize?

I agree with mshurn and shaunup, with some slight differences, based on the emphasis throughout the poem.

I feel the poem is about Frost being content with a big life decision he has made, both of which were equally valid. I could, for example assume that he is referring to his decision to become a poet (road less travelled), and his contentment and fulfillment when looking back at the decision later on.

The yellow woods, and the whole first paragraph certainly set the mood, which i would say is reflective and light.

Obviously the decision is made before the reflection, and i'd say quite a bit further on down the path for him to consider looking back to that critical choice, and because of the light mood and contentment with it, i'd say it was when he realised he was happy with the decision that he'd made.

Therefore, the yellow woods would certainly set the season of his life in which he made the decision.

Autumn would perhaps suggest that Summer had just gone, and it was a time of ending - say to use my analogy from above, a career change, which perhaps brought the need for a choice, while Spring would suggest a new beginning, perhaps his choice in career.

In any case, my focus for this meander is just to highlight that the whole context is about a quiet, light realisation that he has been fulfilled and is content with the choice he made, and the yellow woods are where he was when the choice was set before him.