Target Display Mode is a useful feature for iMac owners who also own other Macs. It allows you to use the iMac’s display as an external screen for the other Mac.
Unfortunately, Target Display Mode is no longer available in the recently released 2017 iMac. As first reported by AppleInsider, Apple confirmed that Target Display Mode has come to an end, and that future iMacs will not have the feature.
Target Display Mode was actually discontinued by Apple on 2014. The 27-inch Retina 5K iMac released in October 2014 was the first model to not support the feature.