AIRPLAY DISPLAY
When your notebook is close to a desktop Mac with its own larger display, you can now use the display of that nearby Mac to extend that on your notebook, provided that they both fully support AirPlay. Both need to be signed into the same Apple ID, and to be able to connect with one another over Wi–Fi or through a USB cable, with Bluetooth turned on. Open the Displays pane on the notebook and enable the other Mac using the Add Display pop–up at the lower left. At first, the image shown on your notebook might require a magnifying glass, but you can then adjust the resolution on each, and whether you want the connected display to mirror or extend that on the notebook.
Wi–Fi is more convenient to use for short periods but has a noticeable lag which all but vanishes if you use a USB connection between the two Macs. It’s more than adequate if you just need the additional display area to show static images or reference documents, but you wouldn’t want to use it to edit a document for too long.