Files are the containers for the rules. They define imported modules (pe, hash, …) and have a list of rules to be included. Before you can create rules, you need to create at least one file to put your rule(s) inside.
YaraEditor allows to create files, define imports needed for them as well as copy them (with all the rules inside), delete them (with all the rules inside) or edit name and imports.
Once a file is created, you can open the file page and have an overview on the rules inside. It allows to modify the file as well as manage the rules inside.