[Python] Can anyone explain me Python for loop (or iteration) algorithm

I am learning python from Coursera . I have written one program , According to that when I click on the screen , it draws circle . See the program below --

# Dots

# importing
import simplegui
import math

width = 600
height = 600
ball_list = []
radius = 20
colour = "Yellow"

# position ditector
def distance(p,q) :
return math.sqrt((p[0]-q[0])**2 + (p[1]-q[1])**2)



# Mouse click -- Change the position
def click(pos) :
ball_list.append(pos)
# global position
# global colour
# if distance(pos, position) < radius :
# colour = "Blue"
# else :
# position = list(pos)
# colour = "Yellow"


# Drawing the ball
def draw(canvas) :
for position in ball_list :
canvas.draw_circle(position , radius , 2, "Black" , colour)

# Creating the frame
frame = simplegui.create_frame("Dots" , 600,600)
frame.set_canvas_background("White")

frame.set_draw_handler(draw)

# mouse click
frame.set_mouseclick_handler(click)


# Start
frame.start()


But my doubt is in def draw(canvas), for position in ball_list , I have not defined any position . I made position = list(pos) as comment . Then what is the value of position in position in ball_list , How can for loop work without any value ? What is iteration ? What is the difference between for loop and iteration ?

If above code doesn't work in your IDE, please go to http://www.codeskulptor.org/#user38_VSZ0jZ0uTh_0.py and run it .

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