How to use external display with IMac27

Hello there

I hope you can help.

I want to use an extra external display together with my Imac27 - I have a MBA that I would like to use, just to have a little extra screen for some things.

How do I set it up, I'm not sure which ports and cables I need to use.

I want to use my Mac book Air computer as a display... I know how to get the display out of that one - but not how to get a display INTO it.

The ports I'm looking at ...you tell me.. I've attached two pictures