A Matrix Templar is an Autobot who has shown a great link with the Matrix of Leadership, transforming them into something beyond what they originally were.
Primus, creator of the Matrix, is the original Matrix Templar, a being of great power who learned how to control his size. His only rival was the original Chaos Bringer, Unicron.
Other Matrix Templars who are well known are Optimus Prime, the "Chosen One" Rodimus Prime, and even the Decepticon Scourge proving you don't necessarily have to be "good" to be chosen by the Matrix.
Matrix Templars are the strongest of the Autobots, capable of growing beyond the limitations of normal Autobots. As such, Matrix Templars are capable of great damage if provoked. Even though Templars prefer the route of peace, they are more than prepared for war.