In iOS 9, Apple introduced the ability for developers to create content blocking extensions for Safari, which have a large number of theoretical uses. However, the primary use for this type of extension seems to be to create blockers for ads and tracking scripts.
In case you haven't heard, there has been a bit of controversy over these types of applications. Most notably, well-known iOS developer Marco Arment (of Overcast, formerly Instapaper), created a content blocker named Peace, which he pulled from the App Store shortly after release, citing a guilty conscience as the reason.