A (physical) file system is a disk or partition, formatted in a certain way, and containing data structures comprising directories and files.
Every physical file system has a root directory indicated as
/ in Linux.
Now this structure maps to a logical view. A simple logical view maps one physical file system one-on-one to your logical file system. This is called 'mounting a file system'.
You can mount a physical file system (or part of it) to a different logical node, called a 'mount point'. This means, your physical file system root
/ can be mounted to your logical "directory"
If your physical file system contains a file
rootfile in its physical root directory, you may access exactly that file by its logical name
The physical location is still on the physical file system, your access through the "mount point" is just a logical link to access that data.
Mount are useful for organizing various file systems, for combining several disks/partitions into one logical structure, and to combine various disparate devices and views into the same structure, from disks, partitions, USB sticks, floppy drives, remote network drives and even terminals, all logically residing under your logical root