Remembrances are dropped by Elden Ring's most significant bosses. They have a basic function as consumable rune items, but more importantly, each can be redeemed by Enia for a reward, your choice of one of two pieces of equipment associated with the boss.
Wandering Mausoleums are crypts on the backs of massive stone turtles that often amble around in a small path. They are covered, either on top or on their feet, in dark magical residue resembling many skulls. To interact with the Mausoleum, you must clear much of the residue off with attacks, until finally it collapses and becomes inert, allowing you to climb inside. Here you can duplicate one of your Remembrances, even if you've used it already, meaning you can redeem both rewards. Only one duplication is allowed per Mausoleum per playthrough. Mausoleums without big bells attached can only duplicate select Remembrances.