GLSL is the lingua franca of real-time graphics outside game engines: a compact C-style language compiled by the graphics driver and executed in parallel across the GPU. Vectors and matrices are native types; functions like mix, smoothstep, and dot are the working vocabulary.
Artists meet GLSL through WebGL, Shadertoy, TouchDesigner, and VJ formats like ISF. The language rewards a particular style of thought: describe the image as mathematics of position and time, not as a sequence of drawing commands.
Its portability is the point. A GLSL fragment shader written for the browser moves into most visual tools with minor edits, making it the closest thing live visuals has to a shared instrument.