Scarborough is a small town on the coast of England. "Scarborough Fair" was a popular gathering in Medieval times, attracting traders and entertainers from all over the country. The fair lasted 45 days and started every August 15th.
The ballad (the lyrics of which probably have their roots in the 17th Century Scottish ballad 'The Elfin Knight') relates the tale of a young man who instructs the listener to tell his former love to perform for him a series of impossible tasks such as making him a shirt without a seam and then washing it in a dry well, adding that if she completes these tasks he will take her back. Often the song is sung as a duet, with the woman then giving her lover a series of equally impossible tasks, promising to give him his seamless shirt once he has finished.
In Medieval times, the herbs mentioned in the song represented virtues: parsley was comfort, sage was strength, rosemary was love, and thyme was courage.