so is this Adventure mode ?

[Life-Binder]

The endgame content can make use of all of the Broken Isles thanks to the scaling.
Legion will have ongoing world quests that are constantly changing. Varied reasons, gameplay, and rewards.
Objectives will last from hours to an entire week depending on how important it is.
The team wants to overwhelm you with variety and choices.
Rather than choosing what missions to send followers on, you can go on your own missions!
Some of the objectives are reputation specific.
Some of the objectives are PvP related, such as assaulting an Alliance tower as Horde.
World Bosses can be an objective that cycles through.
Gatherers will have special objectives for them only.
This can bring back some minigame gameplay, such as the rolling on the wall quest in Vale.
All of the region will have the same objectives at the same time so that you can play with friends on other servers.
Emissaries will send you on a quest to do some objectives for their faction. You can pick from the available ones. These aren't daily quests, they will hang around for a couple days.
If you don't log in for a while, there may be more emissaries waiting for you, similar to Hearthstone.

is this what they meant by AM in WoW:L or is it something else ?

[Isheria]

Have you not done any bounties on adventure mode in diablo 3? It's an exact copy of the system except that the bounties (quests, events) last longer and are randomly generated just like in diablo 3 that's why it's called adventure mode, because it's straight port of the feature from diablo to WoW.