King of the Hill is a gamemode common to first-person shooters. In general, kills net you no points. Instead an area of the map is designated as 'the hill' and you gain points for every second you occupy it uncontested. The first player to a set number of points or the person with the most points at the end of the time limit is the winner.
A slight variation of this is Crazy King of the Hill (usually shortened to 'Crazy King'). The only difference between this and the standard version is that the hill periodically moves around the map.