Cydia Substrate is a software platform that allows developers to create modifications, known as "tweaks," for iOS devices. It provides a framework that enables these tweaks to alter the behavior and appearance of apps and the operating system without needing to change the original code. Originally developed by Jay Freeman, also known as saurik, Cydia Substrate is commonly used in the jailbreaking community. It works alongside the Cydia app store, which is a popular platform for distributing these tweaks, giving users the ability to customize their devices beyond the limitations set by Apple.