What is 6v6?
6v6 also know as 6s is a competive format that is considerd much for serious then highlander, with most unlockable items being banned and many class restrictons.
The teams are divided into six on each team with the usual loudouts being two scouts, two soldiers, one demoman and one medic. The gamemode usually played on this format is control points.
The gameplay also revolves around uberpushes, which the medic build up over time and when active makes the medic and the traget the medic is healing invincible for a short time. A Uberpush can quickly decide the game.