The expert skillcape shard bag is a bag where one can store all their skill shards. Those shards are used to make Expert Skillcapes. This was added with the addition of the second batch of Prifddinas in the Patch Notes. It is automatically placed in Diango's item retrieval interface at log in and is therefore also accessible from the toy box in the player's player-owned house or the item retrieval interface in the bank.
As long as the player has the bag, even if it's in the bank, all skill shards obtained will automatically be placed inside with the message: "You found a shard emanating magical energy as you were training. It has been added to your shard bag."
However if the player stores the bag in their player-owned house (toy box (page 6)) shards in the bag appear to be unnoticed by the game and the player is issued "replacement" shards.