It's the "Sec Ops " occupations of the Army Navy, USMC, Air force Each branch has there own "MI" units. It can involve Investigations, Analysis, Interrogations, Intel gathering operations.
You need to score very well on your ASVAB for MI consideration. ie The higher your line scores the more opportunities / job choices you have. It will also inform you of the physical standards you need to meet for enlistment. Just stay away from enlistment bonuses they are usually to the hard to fill jobs.
Here's a good place to search around