MIL-SPEC is short for Military Specifications. The U. S. military uses standards. Sometimes they conform to say, an ANSI standard, and sometimes the standard is unique to the military - or a particular branch of the military.
Some Mil-Specs are free and others have to be purchased (or gotten through Interlibrary Loan). You can go to ASSIST and if you use the quick search to check for a specific mil-spec (and sometimes it is free .pdf) you don't have to register.