iOS 10.3.1 is out. The release notes don't specify what it fixes that wasn't addressed in the wide-ranging iOS 10.3 update released just a week ago, but we do know that this new update includes bug fixes and improves the security of your iPhone or iPad. Specifically, according to the more detailed notes on Apple's security page, 10.3.1 addresses a buffer overflow that could be exploited to execute code on your phone or tablet's Wi-Fi chip.
The bug is credited to Google's Project Zero, which discloses bugs to the public 90 days after telling companies about them to encourage faster security patches.
Apple released a beta of iOS 10.3.2 last week shortly after releasing iOS 10.3. It will likely go through a handful of additional beta builds and be released to the public in a month or two. We don't expect it to change much, given that the public reveal of iOS 11 in June is just a couple of months away.
Update: The iOS 10.3 update wasn't available as an over-the-air download for the iPhone 5 and 5C and the fourth-generation iPad, the only remaining 32-bit iDevices (it was still available through iTunes). The iOS 10.3.1 update appears to fix whatever the bug was, and 32-bit devices can now see and install iOS 10.3.1 normally.