The Mana Mirror is an item added by Botania. When present in the player's inventory, it will provide Mana from the linked Mana Pool to Mana-using items in their inventory, similar to a Mana Tablet. The Mirror cannot accept Mana in any other way, and will not output Mana when dropped on a Mana Pool.
Shift right-clicking a Mana Pool with the Mirror will link the two together. The Mirror will load the chunk containing the linked Pool.