What is a series
A series is the main group that contains anime and manga. It doesn't contain a lot of data but it offers a view to know every data about a series.
You can find a general synopsis of the series, the original source, and all the global things linked to this series.
What is the use of a series?
It's not really useful for small series which contain only a manga or only an anime but for big series like One Piece, it's really useful, you can find all the manga and anime orders by release date, so for example, you can easily find every One Piece movie!