The question is pretty self explanatory. Personally, i don't think there's any confusion. Assuming of course that you don't read into the instructions and try and make it mean something different.
QUESTION: Please provide any additional information or background you believe will be of assistance in evaluating your application.
Broken down:
1. Please provide any additional information that will be of assistance in evaluating your application
2. Please provide any background of yourself you believe will be of assistance in evaluating your application
So basically, this means that if there is any information, clarification, accolades, achievements, motivation, etc. that didn't have a space to address some place else in the application, then THIS would be your place and opportunity to include it. As wildblueyonder mentioned, don't REPEAT yourself here. Fill it with things you didn't have a place to put somewhere else.
As for leaving it blank; if I read one that was blank, then I would automatically come to the conclusiong that either: A) You had a very uneventful life; or B) You don't have a lot of confidence in yourself or your achievements and don't believe such information is worthy of mentioning. Either way, I would see it as a negative towards your application. If you are doing your application correctly, the problem you SHOULD HAVE, is "I have TOO MUCH information and not enough places to include it". Thus, you have to prioritize your information. If you don't have enough information to include, then that's not a good thing. Especially for the typical "TYPE-A Personality Overachiever" that is the majority of applicants, appointees, and cadets.