It's essentially a $5 coupon with free shipping, $5 will get you one set of the below strings.
Go to this page:
http://www.stringsandbeyond.com/snb-challenge.html

Choose one of the three gauges of strings, put them in your cart, add the code SNBCHALLENGE and they are free, no credit card required, free shipping, etc. While supplies last.

The three possible selections are:

D'Addario EJ26 Phosphor Bronze Acoustic Guitar Strings Custom Light .011 - .052
D'Addario EJ16 Phosphor Bronze Acoustic Guitar Strings Light .012 - .053
D'Addario EJ17 Phosphor Bronze Acoustic Guitar Strings Medium .013 - .056