ISF wraps a GLSL shader in a JSON header describing its controls: this float is a slider, this color a picker, this input a video feed. Host software reads the header and builds the interface automatically, so one file is both effect and UI.
Created by VIDVOX for VDMX, the format spread across the VJ ecosystem, and large public libraries of ISF generators and effects exist to be dropped into supporting apps.
ISF matters because it made shader work shareable at the tool level rather than the copy-paste level, giving live visuals something like the plugin culture audio has had for decades.