Tac-Com is an abbreviated term for Tactical Command. Tac-Coms are usually located in the heart of SRPA bases, as it is where all commanding officers in the area build and execute strategic battle plans, which usually involve Black Ops and Sentinel operations. They are presumably the same command operation who supply intelligence reports to soldiers in the field, such as Nathan Hale and Specter Team.
A Tac-Com has only been shown once, in SRPA 3: it was a large room with a giant multi-paneled computer operating on SrpaNet (SRPA's specialized terminal network), with around 10 people working on the computer. Due to SRPA 3 being located in and under San Francisco Bay, there was also a large thick glass window which gave an wide view of the ocean floor, several squads of Black Ops could be seen, and 6 unknown Sentinels were apparently prepping for combat. Major Richard Blake was the commanding officer in that base, as he was seen directing all military objectives and monitoring all Chimeran movements before a Kraken breached and flooding the room.