What is referred to in the first five lines of the poem?

If you are talking about the following five lines:

Two roads diverged in a yellow wood, And sorry I could not travel bothAnd be one traveler, long I stoodAnd looked down one as far as I couldTo where it bent in the undergrowth; "

Then we can say that the writer remembers being in a yellow wood at fork in the trail. While you read this part you can see that the path is probably away from normal modes of transportation. He suggests in the poem that he will not likely come this way again, further backing the idea this is not a familiar place. The place called "Yellow wood" is named by the idea of fall with the leaves having turned colors away from green.