A lightbox allows you to make a collection of images as you browse the galleries so you can review them later. You can add or remove images at any time. You can also share a lightbox so other people can see your selection.
You don't have to register on the site to use a lightbox, but doing so will allow you to save and manage multiple lightboxes. Registering also saves you having to re-enter details when you make a purchase, lets you view past orders, and so on.