A filesystem is the structure and rule set the operating system uses to organize and access files on storage.
Learners often mix up file, folder, disk, partition, and filesystem. If those words blur together, storage work becomes risky fast.
A filesystem is how the system stores, names, arranges, and tracks files and folders on a disk or other storage device.
A filesystem is the way the computer keeps files organized so it can find them later.
The storage device is the hardware. The filesystem is the organizing system placed on that storage so files can be created, found, and managed.
When you understand filesystem clearly, later lessons about mounts, permissions, corruption, and compatibility become much easier.
Study the larger concept after the word itself becomes clear.