Here is a quick list of the basic functions of a filesystem:
Non-volatile data storage and organization
Provision of a namespace: a system of naming and organizing the constituent files and data
Provision of a metadata structure to support the namespace with hierarchy and directory structure, carrying supplementary information, like file date, size, or location
Use of an Application Programming Interface (API) so that files and directories can be created, accessed, moved, or deleted by the user
Data security and protection from silent corruption and malicious attacks along with access control