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.
The Thunderfire Cannon's four barrels allow it to have, as you can imagine, an impressive amount of firepower that can be used to act as a suppressive form of ordnance and is thus one of the few close range ordnance weapons the Imperium has. Due to their crowd control tendencies, the Thunderfire cannon is able to carry a few specialized rounds for different combat engagements. The first type of shell detonates on surface contact, commonly used against infantry in the open, while the airburst shell is employed against infantry taking cover. The most devastating, however, are the subterranean shells, programmed to burrow into the ground before exploding. The resulting explosion causes seismic disruptions which can knock the enemy off-balance, making them easier targets for follow-up fire. This affects even grav-vehicles, whose anti-gravity generators are negatively impacted by the sub-surface shockwaves caused by the shells.
A Land Raider variant, the Land Raider Achilles, uses the Thunderfire cannon as its main weapon. Yes, you heard that right, the Space Marines have a vehicle that uses another vehicle as a main weapon.