Apple has released updates for iOS/iPadOS, macOS, and watchOS that primarily address bugs and security vulnerabilities.
iOS 15.3.1 is a minor update, feature-wise, for most users. It fixes a problem with Braille displays and addresses an arbitrary code execution vulnerability.
Apple’s iOS update notes are as follows
iOS 15.3.1 provides important security updates for your iPhone and fixes an issue that may cause Braille displays to stop responding.
Additionally, Apple has published information about the security updates on its support website:
WebKit
Available for: iPhone 6s and later, iPad Pro (all models), iPad Air 2 and later, iPad 5th generation and later, iPad mini 4 and later, and iPod touch (7th generation)
Impact: Processing maliciously crafted web content may lead to arbitrary code execution. Apple is aware of a report that this issue may have been actively exploited.
Description: A use after free issue was addressed with improved memory management.
CVE-2022-22620: an anonymous researcher
Meanwhile, macOS Monterey 12.2.1 mainly fixes a bug that could cause Macs to unexpectedly drain battery. Here are the update notes: