An assault gun is a gun or howitzer mounted on a motor vehicle or armoured chassis, designed for use in the direct fire role in support of infantry when attacking other infantry or fortified positions. The term is a literal translation of the German word Sturmgeschütz. Germany introduced the first purpose-built assault gun, the StuG III, in the late 1930s, thus establishing this category of armoured vehicles.
In Milihime, they will be one of your available "artillery" after reaching a certain map. Although not powerful as the others, it is good in maps with few battles as they'll run out of fuel and ammo pretty quick.