Apple has released a new software update for iPhone, iPad, and iPod touch. iOS 11.3.1 is likely a bug fix update to the iOS 11.3 software update released last month. Apple has also released a security update to the recent macOS 10.13.4 update to macOS High Sierra.
Apple currently shows the iOS 11.3.1 release on its developer portal with build version 15E302, and the release will likely be released over-the-air shortly for all customers.
With iOS 11.3 released in late March and iOS 11.3.1 out today, Apple is also running a developer and public beta version of iOS 11.4, likely due out in late May or early June, which includes AirPlay 2 and Messages on iCloud.
iOS 11.3.1 includes a fix for display issues experienced on iPhone 8 with third-party repairs and security improvements for all iPhones and iPads.