Xcode 9.3.1 is a minor bug fix update. According to Apple's release notes, it addresses an issue that could cause a user to have to log in with their Apple ID repeatedly and it fixes a bug that could cause Xcode to hang when using Playgrounds.
Xcode 9.3, the previous update, introduced a new Energy organizer designed to help developers detect when an app or app extension is using too much battery life on a user's device.
The software also brought a new 64-bit testing mode for macOS 10.13.4, which is designed to let developers test software for 64-bit compatibility as Apple prepares to start phasing out 32-bit Mac apps. As of Xcode 9.3, Xcode support for macOS 32-bit support has been depreciated.
Xcode 9.3.1 will be followed by Xcode 9.4, which is currently in beta testing. Apple has seeded two betas of Xcode 9.4 to developers thus far.