To mount, in this context, is to make a group of files in a file system structure accessible to a user or user group. In some usages, it means to make a device physically accessible. For instance, in data storage, to mount is to place a data medium on a drive in a position to operate.
Whether it comes from a partition on the same drive, a new hard drive, media inserted into an optical drive, a NAS (network-attached storage) or SAN (storage-area network) device or some form of flash memory, any file system must be mounted before it can be displayed and its file system read. Only then can the operating system understand the correct format to write files to the medium for the given file system.
Mount points enable reading from and writing to all manner of storage outside an operating system’s file system by maintaining special directories where the information in a number of volume’s file systems can be connected (mounted).
These special, typically empty, directories become the root directories of the mounted volumes’ file system. Any content the mount points may have had is invisible until the mounted volume is ejected. The metadata provided with the volume’s own information allows supporting operating systems to read and interpret the file system on the volume and present its directory structure and file contents in the mount point (now the newly mounted drive’s root).