Apple began including screen sharing nearly 20 years ago in its Mac operating system. If you have multiple Macs in your home or office on the same local area network (wired, wireless, or both), you can observe or control another Mac on that network. Some regular users aren’t aware it exists because they’ve never needed to use it.
It’s not entirely obvious how to set it up or access it as it requires changes on both the Mac to be shared and looking in the correct place on the Mac viewing the other’s screen. It can be handy to use if you have multiple Macs in different places or a Mac without a keyboard and mouse used as part of a home A/V system, or one bereft of head (display) and inputs deployed as a server.
Read more at Macworld.com