When you login to your FM, you will be looking at your filing cabinet.

To open a drawer (directory) and view its contents, click on its name.

A drawer (directory) can contain individual files and/or sub-directories.
You don't need to create subdirectories, but they do help organize your files.
You can create multiple subdirectories within one directory, and each of those subdirectories can contain sub-sub directories.
Your filing system can be as deep or as simple as you want it to be.
Just keep in mind that your directory structure determines the URLs of your files, so a little for-thought on organizing your FM goes a long way.