According to Apple’s release notes, the update fixes a bug with App Tracking Transparency that may have prevented some users from receiving prompts from apps.
This update fixes an issue with App Tracking Transparency where some users who previously disabled Allow Apps to Request to Track in Settings may not receive prompts from apps after re-enabling it. This update also provides important security updates and is recommended for all users.
Security notes for the release indicate that it addresses a memory corruption issue and an integer overflow in WebKit that could both be exploited using maliciously crafted web content. Apple says it aware of a report that these issues may have been actively exploited, so users are advised to upgrade immediately.
The iOS and iPadOS 14.5.1 updates can be downloaded for free and the software is available on all eligible devices over-the-air in the Settings app. To access the new software, go to Settings > General > Software Update.