The loop() function causes draw() to execute repeatedly. If noLoop is called in setup() the draw() is only executed once. In this example click the mouse to execute loop(), which will cause the draw() to execute repeatedly.
y = 100 def setup(): """ The statements in the setup() function run once when the program begins. """ size(640, 360) # Size should be the first statement stroke(255) # Set stroke color to white noLoop() y = height * 0.5 def draw(): """ The statements in draw() are run until the program is stopped. Each statement is run in sequence and after the last line is read, the first line is run again. """ global y background(0) # Set the background to black line(0, y, width, y) y = y - 1 if y < 0: y = height def mousePressed(): loop()