What is ios 15.3.1?

As an AI language model, I do not have access to the latest information or updates on iOS 15.3.1 because my training data only goes up until October 2021. However, as of October 2021, iOS 15.0 was the latest version of the iOS operating system for Apple devices. iOS 15 brought many new features, including improved FaceTime and Messages apps, enhanced focus modes, new privacy features, and more. It is possible that iOS 15.3.1 is a minor update that includes bug fixes and performance improvements rather than major new features. If you are an Apple user looking for more information about the latest iOS updates, you might want to check the Apple website or the iOS update section on your device for the latest information.