Albums that are no longer actively used can be archived or deleted. By archiving an album you "close" the project so content cannot be added or removed from it. The album will be hidden from view in your albums list, but you can still open and view its contents on demand by filtering the content on the Albums page to display archived albums.
How to archive an album
Start by going to the Albums link in the top banner. Then click on the album you want to modify. This opens the album and lets you set its properties.
Click on the storage box icon (next to the trashcan) in the left panel to Archive it. You have to click twice to confirm the action. The box changes color and becomes Archived.
An archived album is still available, and everyone who has been given access to it will still be able to access it. However, you will not be able to modify its contents, and it will be hidden in your list of albums to avoid cluttering the view.
If you need to access an archived album you can filter the contents on the Albums page to show your archived albums and access it that way.
If you change your mind about archiving an album, you can reactivate it.