The guide number of a flash is a rating of the power output measured in meters or feet. The number is handy when trying to compare different flashes when purchasing of one.
After the purchase, the guide number is not that useful since the camera and flash displays will assist you in getting the correct exposure for your picture without you knowing or caring what the guide number is.
Just for comparison: d200 Guide Number of 12/39 (at ISO 100, m/ft) sb910 Guide number of 34/111.5 (at ISO 100, m/ft) This means the the sb910 flash will shoot about 3x further.
---------+---------+---------+---------+ Joseph K Seattle, WA, USA