At its simplest, an is a cryptographic credential—specifically an AES (Advanced Encryption Standard) key—used to decrypt the iOS firmware image file (typically a .ipsw file) that Apple distributes for device updates.
Given Apple's lockdown, how do researchers still decrypt iOS firmware? The process is non-trivial. ios firmware keys
Since extracting these keys requires significant technical expertise and hardware exploits, most users rely on community databases. Other Notable Features macOS Sequoia 15 Release Notes
Understanding the decrypted bootloader is often the first step in finding the "holes" needed to gain root access to the device. At its simplest
: It acts as a "master key" for a specific chip family, ensuring that only authentic Apple software can be decrypted and run on that hardware. Other Notable Features
macOS Sequoia 15 Release Notes | Apple Developer Documentation