Series, ok, I get the concept, but how can I use them?
Series allow collectors to focus only on subsets of a Portal. If you go to a Portal page, you have a search box and also a dropdown with all the Series available. By choosing a series you will only see the corresponding articles.
Series allow you also to understand how complete is your collection (according to what is known and inserted in a given Portal). To see the level of completeness of your collection, check the Series tab in your collection. If you are in Edit mode (click on the pencil icon), you can even choose which series your collect and which ones you want to ignore. The ones you collect will be visible to other users in your Collection.