< What is mounting >
As you know, you can store your data in different physical storage devices, like floppies, CD-ROMs, and hard disk drives. Your hard disk or disks are also very likely split up into different partitions with different filesystems.
If you're migrating to Linux from Microsoft Windows, you're probably used to accessing all your filesystems very easily: you just boot up your puter, go to My Computer, and find all your Windows partitions there immediately. For example, if you have a second hard drive (or a second Windows partition), it automatically appears as D:\ and you can immediately access it. The same goes for floppies, CD-ROMs, digital cameras, and other storage devices - you just plug them in, and you'll be able to immediately access them. However, this isn't the case in Linux.
You're probably a bit confused at first: you put your floppy or CD into the drive and start wondering why you're not able to access it! This is because your floppies, CDs, hard disk partitions, and other storage devices must be attached to some existing directory on your system before they can be accessed. This attaching is called mounting, and the directory where the device is attached is called a mount point.
After the device is mounted, you can access the files on that device by accessing the directory where the device is attached. When you're done and want to remove the floppy or CD or other device, you need to detach, unmount, it before removing it.