What exactly is "Security Clearance"

[insane]

The potential employer is a defense contractor.  However, my line of work has nothing to do with law enforcement nor spook work, so what do they mean when they say "Security Clearance Required"?  
 

[newstart]

For your job it may not be classified work, but you will be able to have access to classified work. So they are going to require you go through an extensive background check and make you pass a poly to get that job. I think its one of the rules the Government give to companys who win there contracts.