The const section declares a constant, which is then available in the rest of the script, including GLSL blocks. Currently, only basic GLSL types are allowed (float, int, vecn, ivecn, matn, and bool). To use a different type, a structure, or an array, you can define the constant inside a GLSL block. Then it will only be available in GLSL code.
const ivec2 RESOLUTION = ivec2(640, 480);