Since the inception of the App Store in 2008, Apple has maintained a "walled garden" approach to software distribution. This closed ecosystem mandates that all applications undergo a rigorous review process and be cryptographically signed by Apple to run on iOS devices. While this model ensures a high baseline of security and quality, it also restricts user freedom, preventing the modification of apps, the sideloading of unauthorized software, and the circumvention of monetization schemes.
Before we can understand an "IPA mod," we must understand the container. An (iOS App Store Package) file is essentially a zip archive that contains an iOS app. It includes the executable code (compiled in Objective-C or Swift), images, plist configuration files, and other resources required for the app to run on an iPhone or iPad. ipa mod