It looks like you're using Internet Explorer 11 or older. This website works best with modern browsers such as the latest versions of Chrome, Firefox, Safari, and Edge. If you continue with this browser, you may see unexpected results.
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.