The theme of this poem centers around choices, and which ones bring us the fullest life as human beings.
In "The Road Not Taken," we see someone in the woods at the fork in the road...pondering which way would be the better one to travel.
The one path is one he has traveled before...so it would be safe, and easy to travel. The other path is unknown, and will be scary to travel because there will be more obstales in the way. The author chooses to take "the road less traveled", and he concludes that this has made "all the difference."
We assume then, that the moral or theme of the poem is that choosing individuality...stepping out of the comfort zone and being a nonconformist...following ones own heart and dreams...is what makes life full and rich.