It’s the master compositor, laying all windows out to generate the image to be displayed. It also handles input and other event routing, as it knows where each app’s windows are, and which are at the front.
SkyLight
A collaborative effort between WindowManagement, Launch Services, SkyLight and other subsystems, there’s deep integration with Spaces.
The master compositor of windows, WindowsServer is a key part of the GUI which runs from when the login window appears until your Mac shuts down.
This vital service composites individual windows to form what will be displayed, and routes clicks/taps to the correct app.
It looks much the same as Sierra. Looking at High Sierra’s logs, though, shows that there are significant internal changes. Here’s a first glimpse at SkyLight, APFS and dasd at work.
The Eclectic Light Company 