OpenGL Shading Language (GLSL) is the language used to write shader programs that run on the GPU — vertex shaders that transform geometry, fragment shaders that compute pixel colors, and compute shaders that do general parallel computation. The same skills transfer to WebGL (which is OpenGL ES with the same shader language) and partially to Vulkan (where the modern Vulkan flavor uses SPIR-V compiled from GLSL or HLSL).
Courses cover the fundamentals — vector math, color spaces, lighting models, transforms — and apply them through procedural noise, raymarching, post-processing effects, and the kind of GPU-driven rendering used in shadertoy.com showcases. The skill is rare enough among generalist developers that it differentiates portfolios for graphics-heavy roles.
