As noted in the release note, the update fixes a vulnerability that may have been actively exploited. Here is Apple’s release note:
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 universal cross site scripting. Apple is aware of a report that this issue may have been actively exploited.
Description: This issue was addressed by improved management of object lifetimes.
CVE-2021-1879: Clement Lecigne of Google Threat Analysis Group and Billy Leonard of Google Threat Analysis Group
The iOS 14.4.2 and iPadOS 14.4.2 software updates can be downloaded from the Setting app on all the eligible devices.