The series page shows your entire library, and allows you to select individual series (however searching can be more efficient in large libraries) and make searches for specific series, as well as being able to edit them. It also allows you to filter your series.
The filters options allows you to narrow your series down, and is incredibly helpful. It can be used to see release dates, names, episode counts, disk size counts, and too much more to list, including custom filters, to fit your every need. These can also be used in the mass editor.
The add new feature allows you to add a new series for Sonarr to monitor and download.
Library Import allows you to import existing organized series and episode files into Sonarr via existing files in the path directory. This is especially useful when making a new Sonarr instance and wanting to keep your existing series.
Library import is for adding and importing an existing organized library of series into Sonarr
Library Import shall not be used for:
The mass editor allows you to edit series in mass. Such as changing any of the previous settings made when you added the series.
Information about how many seasons every series has and how many episodes in each season are missing.