A collection is a set of images on a specific theme created with the purpose of easier access to those images in the future. Instead of searching for them each time or keeping a list of URLs, you can create a collection/lightbox for future use. The feature requests the owner (the person who creates it) to assign relevant title, keywords and description. They should be relevant as other designers could use the collection in the future. Dreamstime.com is a community so share this experience even if you are creating the collection/lightbox for your own use.