Attunements are specific quest lines or actions that players should accomplish to get rewarded with access to a particular heroic dungeon or raid. Burning Crusade was the first expansion that made them the key element of gameplay. Some TBC attunements are long, require a reputation grind, and can take up to 7 days to finish.
There are the following attunements in the 1st Phase of TBC Classic:
Karazhan Raid Attunement;
heroic Auchindoun Attunement;
Shadow Labyrinth unlock;
heroic Caverns of Time Attunement;
Black Morass Access unlock;
heroic Coilfang Reservoir Attunement;
heroic Tempest Keep Attunement;
Arcatraz Access unlock;
heroic Hellfire Citadel Attunement;
Shattered Halls Access Unlock.
70 level TBCC character;
WoW Classic: TBC expansion active;
this boost is piloted;
no specific gear required.