Unix systems have a single directory tree. All accessible storage must have an associated location in this single directory tree. This is unlike Windows where (in the most common syntax for file paths) there is one directory tree per storage component (drive).
Mounting is the act of associating a storage device to a particular location in the directory tree. For example, when the system boots, a particular storage device (commonly called the root partition) is associated with the root of the directory tree, i.e., that storage device is mounted on
/ (the root directory).
It's worth noting that mounting not only associates the device containing the data with a directory, but also with a filesystem driver, which is a piece of code that understands how the data on the device is organized and presents it as files and directories.
Let's say you now want to access files on a CD-ROM. You must mount the CD-ROM on a location in the directory tree (this may be done automatically when you insert the CD). Let's say the CD-ROM device is
/dev/cdrom and the chosen mount point is
/media/cdrom. The corresponding command is
mount /dev/cdrom /media/cdrom
After that command is run, a file whose location on the CD-ROM is
/dir/file is now accessible on your system as
/media/cdrom/dir/file. When you've finished using the CD, you run the command
umount /dev/cdrom or
umount /media/cdrom (both will work; typical desktop environments will do this when you click on the “eject” or ”safely remove” button).
Mounting applies to anything that is made accessible as files, not just actual storage devices. For example, all Linux systems have a special filesystem mounted under
/proc. That filesystem (called
proc) does not have underlying storage: the files in it give information about running processes and various other system information; the information is provided directly by the kernel from its in-memory data structures.