Coordinates.

// Sets the screen to be 640 pixels wide and 360 pixels high
size(640, 360);

// Set the background to black and turn off the fill color
background(0);
noFill();

// The two parameters of the point() method each specify coordinates.
// The first parameter is the x-coordinate and the second is the Y 
stroke(255);
point(width * 0.5, height * 0.5);
point(width * 0.5, height * 0.25); 

// Coordinates are used for drawing all shapes, not just points.
// Parameters for different functions are used for different purposes.
// For example, the first two parameters to line() specify 
// the coordinates of the first endpoint and the second two parameters 
// specify the second endpoint
stroke(0, 153, 255);
line(0, height*0.33, width, height*0.33);

// By default, the first two parameters to rect() are the 
// coordinates of the upper-left corner and the second pair
// is the width and height
stroke(255, 153, 0);
rect(width*0.25, height*0.1, width * 0.5, height * 0.8);

Functions Used