what does mod mean in python code example

Example: mod in python

x1 = int(input())
y1 = int(input())
x2 = int(input())
y2 = int(input())

if( ( (x2-x1) % 2 = 1 and (y2-y1) % 2 = 1 ) or ( (x2-x1) % 2 = 0 and (y2-y1) % 2 = 0) ):
    print('YES')
else:
    print('NO')