GLSL
The language WebGL shaders are written in, close to C with vectors built in.
See it in Shader forge. Pick the Voronoi cells preset and read the code beside it.
In plain words
GLSL is short for OpenGL Shading Language. It is the language for the small programs that run on the graphics chip in WebGL. It looks like C, with types such as float for a number and vec3 for three numbers. Maths on positions and colours is built in, so one line can mix two colours. A GLSL program cannot open files or talk to the page; it only works out numbers.