Memory Rooms are secret rooms that can be found on random levels.
Memory rooms are found by following The Archivist until he walks into a wall. The memory rooms are full of objects that you can search through if you walk up to one and hit 'F'. The objects can have either lore pages, items, juice boxes, Edgewood Meritorial Tokens or nothing in them. Each lore page is linked to an achievement, however you do not need the page to unlock the achievement (ie: you can get the "Die in Edgewood" achievement long before finding the lore page associated with it).
The ArchivistEditThe Archivist is a ghostly apparition that appears randomly on a level with a memory room. You will see him standing in the middle of a room when you enter. He will then walk off towards another door. After beating the room follow him. Eventually he will walk through a wall. This is the wall with the secret room. Clear the room, then follow him through the wall. Also note that once you leave the memory room, it cannot be entered again.
Sometimes he will be in the secret room alone and will then attack the player. He walks towards the player and then stops and launches two waves of paper that are easily avoidable. Like other champion enemies, Archivists drop an item upon death. Killing an Archivist completes the You Belong In A Museum achievement, unlocking the Edgewood Plans item.
Sometimes, The Archivist will only go to the right when walking into walls. If this occurs, he has bugged out and a memory room will not be found. This is the case even if you have the Recall perk class activated.