The filestore provides secure, encrypted storage for files uploaded to the platform, primarily via form submissions.
It's not something you should need to work with directly. Files are placed into the filestore by other platform services, and references are held to those files in form sessions, history, workflow or object data. When these services have all finished with a file, the file is deleted.
For example, you may have a file in the filestore, and a single reference to it held in a history. When that history is deleted via the History API, the file is automatically deleted too.
The File Store worker provides an API that not only stores and returns files, but keeps track of those files via dedicated database tables.
The Handling File Uploads article looks at the file store in more detail, and how reference to files are used by workflow, history and iCM objects.