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Apple has just released iOS 15.3.1 and iPadOS 15.3.1, which fixes a small issue that could cause Braille displays to stop responding.
More importantly, it includes a fix for a critical Webkit vulnerability that may have already been used in the wild. The security bulletin describes the bug this way:
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.
Webkit is Apple's web rendering engine, and it is required to be used by all browsers on iOS and iPadOS, not just Safari. This means that no matter what browser you use on your iPhone or iPad, it's possible for users to craft web content in such a way that it allows them to run anything on your device. And worse, it may have already been used in real world, rather than just discovered by security researchers.
In other words, you want to get this patch as soon as possible. To do so, open Settings, tap General, then Software Update.