Some weapons require two hands to use in close combat. These weapons usually have a long shaft. Note not all Great Weapons require two hands to use and are sometimes known as 'double-handed weapons' for that reason. However, Great Weapons are not the only weapons that require two hands to use.
If a weapon requires two hands to use then it is not possible to simultaneously employ a shield or another weapon. If a shield is carried then it must be slung across the warrior's back or dropped behind him whilst he fights in close combat.