The principal business code is a 6 digit business code that describes the kind of work that you do. It is used as a method for the IRS to classify what type of business you have. You can look up your principal business code by clicking on this link and navigating to pages 17 and 18. Pick the code which best fits what you do. It doesn't have to be a perfect fit.

 http://www.irs.gov/pub/irs-pdf/i1040sc.pdf