These updates deliver a range of security patches, addressing vulnerabilities that variously allow arbitrary code execution, privilege elevation, and notionally-unauthorised access to data (including browser history and possibly web credentials).
At least one of these issues is reportedly being actively exploited.
Apple's release notes make no mention of any new features in iOS 15.3, iPadOS 15.3, and Monterey 12.2. The numbering of these updates suggests the company is particularly keen to see users adopt them, and they are "recommended for all users."
The Universal Control feature promised for Monterey and iPadOS 15 is not included in the latest updates, but is still expected this autumn. Universal Control will allow a Mac's mouse and keyboard to control an iPad.
Similarly, tvOS 15.3 and watchOS 8.4 provide bug fixes and security patches, but no new features.