Mythic Keystone Dungeon is similar to the Challenge Gold in the old expansion, established by the Mythic Dungeons. Completing a Regular Mythic dungeon will reward you with a keystone. This keystone will let you start a relevant level Mythic Keystone Dungeon, and get a reward by finishing the Mythic Dungeon. The Mythic Keystone Dungeon is "depleted" if not completing in a provisions time, and can not get Keystone anymore in the week. Since there is no CD limit of playing Mythic Keystone Dungeon, you can always continue, until your Keystone run out.
The time you used to complete the Mythic Keystone Dungeon decide how many levels your Keystone upgrade and loot chests you reward. The best reward is that player gains three chests reward and upgrade Keystone by 3+ levels. And group members all reward loot chest from current mythic level, and level +1, level +2. You will acquire gears in the corresponding Keystone level. There is a level limit of the gears from loot chest, same as the ilvl880 gears from the current Mythic difficult instance.
For each Mythic Keystone Dungeon, the difficulty changes so that all enemies will increase their health and damage. The higher Keystone level, the higher demand for gears and skills of the player. Mythic Keystone Dungeon can't quit when to start, so just do what one is capable of.
The system will reset and settle scores of the Keystone challenge, and reward a chest according to your records of the highest difficulty in the past week. So, play the Mythic Keystone Dungeon every week as far as possible to reward loot chest next week, even don't care the Keystone level.
Completing all the Mythic Keystone Dungeons will gain Broken Isles Keymaster, and unlock an appearance on Artifact Weapon. Also, will reward Feats of Strength by first completing a level 15 keystone. 
Comparing with the Challenge Gold in the previous expansion, Mythic Keystone Dungeon has no level limit that you may never know where your limit is. It is an excellent way to challenge yourself by doing Mythic Keystone Dungeon! And the loot gears which same as the drop from the mythic raid are a great temptation.