Code Python

Can you please help me with the Code in the red rectangle part that I do not understand? Can you explain please? Thanks!

Write a program that performs a circular permutation of given shift in a text box.

The answer that I do not understand the rectangle part, can you please kindly help me here?

$```$

p$1=$ Picture$("$ImagesPython$/$HarryPotter$.$jpg$")$

p$2=$ copy$($p$1)$

i$1=$ Entry$($width$=10)$

b$1=$ Button$($width$=10)$

borneX, borneY $=$ p$1.$width$(),$ p$1.$height$()$

nbPixels $=$ borneX $*$ borneY

def b$1\_$click$()$ :

decalage $=$ int$($i$1.$get$())$

for x in range$($borneX$)$ :

for y in range$($borneY$)$ :

coul $=$ RGB$(*$getRGB$($p$1,$ x$,$ y$))$

nb $=$ int$($borneX $*$ y $+$ x $+$ decalage$)\%$nbPixels

xn $=$ nb $\%$ borneX

yn $=$ nb $//$ borneX

pixel$($p$2,$ xn$,$ yn$,$ coul$)$

$```$