What is surface.blit() function in python ? what does it do? how it works?

I am a beginner in Python and i am not clear about the function surface.blit() method. what does it do? how it works? i have come accross the following points as to how to create it.

• Create a canvas of desired size
• Create a surface of smaller size containing the object to be displayed.
• Define the Rect value of the surface.
• Blit (overlap) the surface on the canvas at the rect position

why only rect is used can it be any other shape