When Apple released iOS 11.3 last month, it contained a number of new features, but there was also a bug that rendered touchscreens unresponsive if they had been replaced by a third-party vendor. Today, the company released iOS 11.3.1, fixing the issue for iPhone 8 users.
Of course, there are also some security fixes in the update, which have to do with Crash Reporter, LinkPresentation, and two that have to do with WebKit. For example, a user could cause UI spoofing with a maliciously crafted text message. And yes, we have seen numerous times before that bad things can happen with the wrong text message.
There's also a security update rolling out for macOS 10.13.4 High Sierra. This fixes the same issues with Crash Reporter and LinkPresentation. And finally, Safari 11.1 gets a security update for the two WebKit security flaws.
iOS 11.3.1 doesn't contain any new features, which should be apparent from the version number. You can update your device by going to Settings -> General -> Software Update.