Last week, Apple released a beta of iOS 10.3.2 a day after it offered 10.3 to the public, leading many to speculate that the company had skipped 10.3.1. As it turns out, it was working on a security fix for the OS, and iOS 10.3.1 was released to supported devices today.
The vulnerability that it fixes was first spotted by Gal Beniamini of Google's Project Zero, and it allows someone to "execute arbitrary code on the Wi-Fi chip". Apple says that it used improved input validation to fix a stack buffer overflow.
Other than that, there doesn't seem to be anything new in the iOS 10.3.1 update. The version number indicates that there aren't any new front-facing features, and Apple only listed the one security fix on its website. After all, it's only been a week since the company released iOS 10.3, which did include a few minor features.
iOS 10.3.1 is available to supported devices, which include iPhone 5, iPad 4th generation, and iPod Touch 6th generation or later. You can grab it by going to Settings -> General -> Software Updates.