This page discusses the concept of ejection. For the UNIX/Linux command click this link.
eject is the opposite of insert
To eject something (usually a cd, dvd or floppy) means to remove it from the computer, not access the content, and usually replace the device to read more data.
You can eject by either pushing a button on the drive or virtually. In linux you need to unmount the device to eject it, and the eject command does this. Most user friendly distros treat this as the same deal.
It is a good idea to eject media before a reboot so that the computer doesn't try and install from the device.