When OpenGL or WebGL interact with a GPU, they pass in data to tell the GPU the geometry and textures it needs to render. At this point, the GPU needs to know how it must render those textures and the geometry associated with them into a single 2D image that will be displayed on your computer monitor. OpenGL Shader Language (GLSL) is a language that is used with both OpenGL and WebGL to instruct the GPU on how to render a 2D image.
Technically, WebGL uses the GLSL ES shader language (sometimes referred to as ELSL), which is a subset of the GLSL language. GLSL ES is the shader language that's used with OpenGL ES, a mobile-friendly subset of OpenGL (the ES is for Embedded Systems). Because WebGL is based on OpenGL ES, it inherited the GLSL ES shader language. Note that whenever I refer to GLSL within the context of WebGL or WebAssembly, I am...
