how does .top work pygame code example

Example: pygame example

1 # Simple pygame program
 2 
 3 # Import and initialize the pygame library
 4 import pygame
 5 pygame.init()
 6 
 7 # Set up the drawing window
 8 screen = pygame.display.set_mode([500, 500])
 9 
10 # Run until the user asks to quit
11 running = True
12 while running:
13 
14     # Did the user click the window close button?
15     for event in pygame.event.get():
16         if event.type == pygame.QUIT:
17             running = False
18 
19     # Fill the background with white
20     screen.fill((255, 255, 255))
21 
22     # Draw a solid blue circle in the center
23     pygame.draw.circle(screen, (0, 0, 255), (250, 250), 75)
24 
25     # Flip the display
26     pygame.display.flip()
27 
28 # Done! Time to quit.
29 pygame.quit()