When you create a new Collection, it will be empty to start with. You can either add an automated rule to add articles to the Collection or you can add articles manually.
Adding a rule
Two types of rules are available: Either you select the rule based on publications which means that all articles from a certain publication will be added to your Collection. Or you select the rule based on keywords which means all articles containing a keyword that you've defined will be added to your Collection.
You can also manually select certain articles to be added to a Collection of your choosing. Simply go to the article full text view of the respective article and click "Add to Collection".