The Thunderfire Cannon is a large minigun on treads that is usually accompanied by a Techmarine of the Adeptus Astartes. The Thunderfire Cannon is a special type of Imperial ordnance that, instead of lobbing shells at the enemy, uses lots and lots of bullets to bog down mass amounts of infantry at close range. You may wonder, with all of their biological augmentations and enhancements, why the SPESS MEHREENS even need to have a mobile weapons platform. The answer is that the cannon is fucking huge with a ridiculous amount of recoil that not even the super enhanced muscles of the Space Marines can hold and fire accurately. It is also used in terrain where mobile artillery platforms (such as Whirlwinds) cannot operate, or when a defensive emplacement must be maintained.
Owing to their relative sophistication, techmarines are given control of these weapons, following them like a murderous puppy. The cannon has a degree of autonomy but nothing on the level of a Cybernetica robot. That said, one Thunderfire Cannon was left in a defensive position, eliminated all enemies, and then took it upon itself to go on a hunter-killer role against a huge army of cultists, chaos marines, and daemons.