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 thus can also be accessed from the toy box in your player-owned house.
Note: As long as you have 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 you store the bag in your player owned house, toy box (page 6), shards in the bag appear to be unnoticed by the game and you are issued "replacement" shards.