When viewing a Collection you have the option of sorting it by Date Added, Date Created, and Last Updated. You can also choose to view the Collection in Grid or List format. All these options have URL params which force those options in case you want to share a Collection in that exact state. Example:

https://codepen.io/collection/AZvoMN?sort_order=desc&sort_by=item_updated_at&grid_type=grid

Collections also have natural order by default, which is set by the creator of the Collection. Pens are in the order that they were added to the Collection in by default, and can be re-organized by the owner any time.

Items in a Collection have an “Order”, which sets their position in the default view of the Collection. The first Pen in a Collection is at position 0. The owner can move items around in the Collection by dragging them to a new position (in list view only), or editing their “Order” number manually.