Q: Mirror

Target display mode if the two Macs support it:

Target Display Mode: Frequently Asked Questions (FAQ) - Apple Support

For the future, this is the Mac Pro desktop forum

If you want the same picture on each, you can open the MacBook's screen in a window on the iMac using Screen Sharing.

On the MacBook:

System preferences > Sharing > Screen Sharing

On the iMac:

In a Finder window, navigate to Shared items, and then to a screen showing the MacBook. Click (Share Screen...), then enter credentials that would allow you to log in at the MacBook.

Once connected, you can Control-click on the Icon in the Dock and choose Option > "Keep Item in Dock".