Feral Ork Mini-Nobz. In a nutshell.
More specifically, a Brute is one of the bigger and nastier Orks in a Feral Ork tribe and usually they operate in groups known as Brute Squads, though they are still not as large or powerful as the Nobz. They spend their time maintaining order within the hierarchy of the tribe, usually by means of their sheer size and domineering tendencies. They allow the other smaller Orks the privilege of hunting for them, taking the best of the food that the Huntas bring back.
Whilst the ability to shoot prey is essential to a Feral Ork tribe's survival, they still relish close quarters combat. Brutes are also used as Minderz by the tribe's or WAAAGH!'s Warboss. They have special orders to keep an eye on a Wyrdboy to make sure he does his job in battle. In such circumstances a Brute's bullying nature proves to be of its greatest use.
Brutes wield giant stone or metal axes, spear and maces to crush anyone smaller than them.
Most converters would use Orruk Brutes from the Age of Sigmar line due to their similarity in name and the fact they look exactly like the Feral Ork equivalent of a Nob. In fact, a lot of Feral Ork players would just use the ones from Fantasy as Count as due to Fantasy Orcs' lower tech level.