While exploits and bugs are found every now and then, it seems that this is a particularly dangerous exploit because according to Apple, it appears that hackers are already taking advantage of this exploit in the wild. This means that this isn’t some theoretical hack, but one that is actually being used, which explains why Apple is urging users to quickly update their devices.
Apple also usually seeds its updates to developers first for beta testing and also to check for app compatibility, but it seems that this situation is urgent enough where Apple sent it straight to release. Interestingly enough this only affects the mobile version of Safari because the macOS version of Safari isn’t affected by this exploit, so Mac users don’t have to worry about this particular issue.
If you don’t see the notification yet, just go to your iPhone/iPad’s Settings, go to General, and then tap on Software Update to manually check for and pull the update.