Metas are meta-events, huge chains of events that some maps have, and most of them also give really good rewards. HoT metas (which means meta-events that take place in Heart of Thorns maps) are some of the most elaborate, and they also give pretty good rewards. The Auric Basin meta in particular gives tons of loot, and it’s pretty fun. All meta events require lots of players working together, though, so it’s better to use the LFG tool to find groups doing the metas.
Living World Season 3 maps. Each of the episodes in Living Season 3 added a map, each map with its own masteries and meta-events and resources. Some of them are really fun (Bloodstone Fen is one of my favorite maps in the whole of Guild Wars 2).
Meta-events in the maps added by Path of Fire. They’re not nearly as extensive and elaborate as the Heart of Thorns metas, but they can be pretty fun - especially the Augury Rock meta-event that you’ll find in Elon Riverlands, and the Maws of Torment meta-event in the Desolation.
Fractals are an evolution of the dungeons in the main game, but yeah, they’re dungeon-like things that require a full team of 5. I don’t do fractals much, but I plan to eventually. I tried a few times and most of those times were pretty fun.
Yeah, all experience you accumulate after level 80 will go to “training” the mastery you selected. You’ll also need Mastery Points to actually unlock the masteries themselves (after they’re fully trained), and you get those via achievements, exploration, and some kinds of challenges, depending on the area.