The term comes from the time when PC had floppy disc drives. If you pressed a button (the eject button) then the disc would pop out of the drive.
The term "Eject" is now used to mean disconnect other devices from the USB port of PCs, such as MP3 players, memory sticks, cameras and iPods.
You will find an "Eject" option in your iTunes application on your PC. Click on this before disconnecting your iPod. This ensures that all the data iTunes has for your iPod is safely stored. Once the eject operation has finished you can disconnect your iPod and PC.