A series is an organizational structure that allows you to organize a group of stories with a common theme or common main character.
Series can be ordered or un-ordered. Ordered series are for a series of stories that are best read or presented in a particular sequence. Un-ordered series is simply a collection of stories and are presented to the reader in alphabetical order.
For example two separate stories that have the same main character who is a detective. Two different cases, that makes two different stories, but it's one series of stories about the detective's exploits.