in Windows your separate each drive by a letter like C:, D:, etc, however on Linux your drives are mounted as part of your folder structure. the top level is called root which would be / you can then mount each disc directly as a folder under root, so for example /home could be a separate hard drive but it’s still mounted under root, note the starting slash. This means the command deletes any and all files+directories under root, this can include mounted USB, mounted network drives and anything mounted to your root. your basically nuking all the files you can access when you’re logged in as admin.