Civilians are units that not part of Warband. As such, they are poorly equipped in combat and have lower chance of survival when your village is under attack.
They started off as a Villager who, if not assigned to perform any tasks, will gather some food in the tile they stationed at. In order to assign them to certain occupation (be either Sailor, Warrior, Healer, etc.), you need to build some Buildings associated with the occupation in question. For instance, Healer's Hut is built to assign one of the villagers to become a healer. It's important to build at least one House per occupied tiles to prevent overpopulation.