Simple fragment shader for beginners with explanations

shader.frag

// # = Preprocessor macros
#ifdef GL_ES // OpenGL Embedded systems needed for openGL on web and mobile devices
precision mediump float; // level of precision for float https://stackoverflow.com/questions/13780609/what-does-precision-mediump-float-mean
// Lower precision means faster compilation but lower quality, above sets all floats to medium precision
#endif

void main() // Main entry point function like c/c++
{
  // gl_FragColor = built in global variable to determine pixel color 
  // werid because you don't see it imported or extended it just exists
  // vec4 = 4 dimensional vector vec expects float by default, other types can be done like so:
  // bvec = boolean vector, ivec = integer vector, dvec = double vector, but float by default
  // Full list of built-in variables can be found here https://www.khronos.org/registry/OpenGL-Refpages/
  gl_FragColor = vec4(0.2, 0.3, 0.1, 1.0);
}