You don't need to know anything about Gen VI or VII, except maybe their settings. While the whole event is themed around tropical Alola and it's Legendaries, the events and users stick to it to as much as they want. Some more, some less. But to put it simple... On the forum (not in the games) Tribes are groups that compete against each other, Kahunas are the leaders of these groups and Z-Crystals are points that each group collects.