The translate() function allows objects to be moved to any location within the window. The first parameter sets the x-axis offset and the second parameter sets the y-axis offset. 

x, y = 0, 0
dim = 80.0


def setup():
    size(640, 360)
    noStroke()


def draw():
    global x, y
    background(102)

    x = x + 0.8
    if x > width + dim:
        x = -dim

    translate(x, height / 2 - dim / 2)
    fill(255)
    rect(-dim / 2, -dim / 2, dim, dim)

    # Transforms accumulate. Notice how this rect moves
    # twice as fast as the other, but it has the same
    # parameter for the x-axis value
    translate(x, dim)
    fill(0)
    rect(-dim / 2, -dim / 2, dim, dim)