Photoshop Server has been in Photoshop for quite some time already, but not many people are familiar with it.
The whole concept of Photoshop Server is to allow a user to make use of the full desktop version of Photoshop and its capabilities, in order to do advanced edits on Photoshop Express for mobile.
First of all, it's important to note that the Photoshop Server feature is currently only available for iOS mobile devices, such as the Apple iPad and iPhone.
If you wish to make use of this feature, you will have to make sure both your desktop computer and iOS device are connected to the same network. Then, make sure Photoshop is open on both the desktop computer as well as Photoshop Express on your mobile device.
Then, head over to Edit > Remote Connections in Photoshop on your computer. You will then create a server name of your choice, as well as a password. Head over to your mobile device, then in Photoshop Express, choose one of the effects you like in the PS Effect submenu. Photoshop Express will automatically open up the Remote Connections options, and you can simply sign into the server you just created.
Once you have successfully connected Photoshop with Photoshop Express, Photoshop will import your mobile project file and automatically start running the action in order to edit your image.
Take a look at the video below by the YouTube channel, PiXimperfect, on What Is Photoshop Server?