Today, Apple released iOS 13.5.1, iPadOS 13.5.1, and watchOS 6.2.6. They're security updates, so there aren't any new front-facing features to play around with. Since iOS 13 was released just over eight months ago, this is the 14th update that it's received.
As for what's actually being fixed, that's mostly still up in the air. At the time of this writing, Apple's security updates page still doesn't show anything new for today.
According to 9to5Mac, there was a zero-day kernel vulnerability that allowed for a new jailbreak tool to work, so that might have been patched. There's also a random reboot issue in iOS 13.5, which can be caused by an issue with certain MP4 video files.
Whether those things have been fixed or not, this is an important update that Apple recommends you install. It's the first update after iOS 13.5, which provided new features such as the new Exposure Notification API that allows for COVID-19 contact tracing. It also makes unlocking your iPhone a lot easier if you happen to be wearing a mask.
Update: Apple has published the security content of the new updates, which also include a macOS supplemental update and tvOS 13.4.6. As it turns out, these updates are all about the jailbreak listed above.