Mythic+ (plus) is the newest remake of Mists of Pandaria and Warlords of Draenor challenge modes.
They differ from what you are used to: item level is not scaled anymore and you do not need to accomplish achievement faster than others, new way is to compete with time itself! In Mythic+ mode quantity of dungeons passed are counted, not the speed. You'll get rewards for each level and the further you go the better loot you get and your Artifacts gain more and more power, item level is increasing significantly. By the final of each week players with the best results will get fantastic jackpot in their Class Order Hall in the form of a Loot Chest! Mythic+ is incredibly difficult mode, mobs and bosses obtain new abilities and effects and these characteristics change every week in order to influence the optimal setup dungeons.