When you will make an attack (Resource Harvesting Tiles, Villages, Castles, Monsters) or participate in Alliance Armies, along with the heroes you also deploy brigades. As with heroes, deploying the brigade is what allows you to take soldiers to the attack. Each brigade will bring a certain amount of soldiers.
You will have more brigades to deploy when you upgrade the Barracks, and when you upgrade the grades of Lorelei (Ranged Brigades), Atlas(Cavalry Brigades) and Gargantor (Infantry Brigades). There are a few researches that give you more brigades to deploy as well: Ranged Command (Elven), Infantry Command (Dwarven) and Cavalry Command (Human).