The "end game" is a term used to describe the play style, game features, and player behavior engaged in by expert players with high level characters.
At that point in the gaming experience, high level characters have completed all of the quests and other challenges, and players are no longer entertained simply by grinding through the normal game. Players want greater challenges at that point, and many turn to item finding, PvP dueling, or other non-traditional occupations.
Many game designers end up producing most post-release content for the end game, since that's where most players are spending their time, rather than simply making new level 1 characters and going through the whole experience again. The Event, added to Diablo II in the v1.11 patch, is a good example of added end game material, since it was only available on the realms, and only possible to engage in with very high level, well-equipped characters.