A warboss is an ork military and political leader. Leadership in ork communities is typically determined by strength, so the largest, strongest, and fiercest ork is usually found in the role of warboss. Cunning is also respected by orks, so the warboss is likely to be relatively smart, as well.
Exceptional warbosses are able to organize large numbers of orks, minimizing internal conflict and directing a massive number of orks against an external target. Such mass movements of orks against other civilizations are known as "Waaaaaghs", and they represent the primary threat posed by orks against the Imperium of Man and other WH40K factions. Eliminating the warboss leading a Waaaagh is usually the swiftest and surest way of stopping the ork invasion, as the orks will usually return to fighting among themselves without strong leadership.