Look at The Book of Shaders, there is a demo, link here
https://thebookofshaders.com/edit.php? The log=160509131554
The along while also didn't understand, my own if you want to draw a rectangle, I wrote as follows:

Float the rect (in vec2 st, vec2 in size) {
//size=- size 0.25 * 0.25;
//the size as incoming is the starting point of the rectangular coordinates,
Vec2 uv=step (size * (1 - size), st * (1 - st));
Return the uv. X * uv. Y;
}

Can this demo I also according to the above ideas take thinking at first, but think impassability, demo inside what size parameters of the mean, why are there

Size=0.25 - size * 0.25;

This code, what do you use? Have a great god can some directions