Collections help you quickly filter a frequently used set of documents on your content list page by tag and/or mask (aka Custom Type)..
It's pretty simple to use. In Settings -> Collections, you can create a new collection.
When you create a collection, specify any custom types you want to filter by and any tags the documents must have. Once set, you will find the collections filter on your Content page.
When selected, all your pre-set filters will be applied.