The software offers a number of improvements to mitigate the “Spectre” vulnerability that was first publicized last week. The other vulnerability, “Meltdown,” can also affect iOS devices. Apple said that iOS 11.2 contained mitigations for that issue. Since both issues are hardware-based, it requires software patches to mitigate any possible effects. There have been no known exploits of either in the wild.
Here’s more about the update in an Apple security document: