What is an expansion cord and what is it used for

Are you sure that you don't mean "expansion card?" If that is what you mean, then here are some definitions found on the following link. define:expansion card - Google Search

Definitions of expansion card on the Web:

• A circuit board that plugs into a motherboard, used to add new peripherals to a computer system. Also called an adapter card.
  www.micro2000uk.co.uk/hardware_glossary.htm
• A special purpose circuit board that can be inserted into your computer to give it additional capabilities. See for example network interface card.
  www.mysouthwest.com.au/Business/SmallBusinessIT/Glossary
• A circuit board inserted into a slot on the motherboard to enhance the capability of the computer.
  www.build-your-own-cheap-computer.com/computer-terms.html
• A card that can be fitted into an expansion slot within a computer to improve a part of its performance.
  www.sitechooser.co.uk/Glossary.htm
• An expansion card (also expansion board, adapter card or accessory card) in computing is a printed circuit board that can be inserted into an expansion slot of a computer motherboard to add additional functionality to a computer system. ...
  en.wikipedia.org/wiki/Expansion card