What is histogram sliding?

import cv2 # importing opencv libraries
from matplotlib import pyplot as plt # importing plotting library
  
image = cv2.imread('/images/img5.png') # reading an image
plt.subplot(2, 1, 1) # creating subplot to display image
plt.title("Image") # allocating title to the plot
plt.imshow(image) # diplaying image
plt.subplot(2, 1, 2) # selecting second subplot
plt.hist(image.ravel(),256,[0,256]) # histogram plotting
plt.title("Histogram", x=0.9, y =0.8) # allocating the title to the plot
plt.show() # displaying the plot

import cv2 # importing opencv libraries
from matplotlib import pyplot as plt # importing plotting library
import numpy as np #importing numpy library
def histogram_sliding(img, shift):
    fill = np.ones(img.shape, np.uint8) * abs(shift)
    if shift > 0:
        return cv2.add(img, fill)
    else:
        return cv2.subtract(img, fill)
image = cv2.imread('/images/img5.png') # reading an image
image1 = histogram_sliding(image, -30) # shifiting histogram to leftwards
plt.subplot(3, 2, 1) # creating subplot to display image
plt.title("Image") # allocating title to the plot
plt.imshow(image1) # diplaying image
plt.subplot(3, 2, 2) # selecting second subplot
plt.hist(image1.ravel(),256,[0,256]) # histogram plotting
plt.title("Histogram") # allocating the title to the plot
plt.show() # displaying the plot
plt.subplot(3, 2, 3) # creating subplot to display image
plt.imshow(image) # diplaying original image
plt.subplot(3, 2, 4) # selecting second subplot
plt.hist(image.ravel(),256,[0,256]) # histogram plotting
plt.show() # displaying the plot
image2 = histogram_sliding(image, 30) # shifiting histogram to rightwards
plt.subplot(3, 2, 5) # creating subplot to display image
plt.imshow(image2) # diplaying image
plt.subplot(3, 2, 6) # selecting second subplot
plt.hist(image2.ravel(),256,[0,256]) # histogram plotting
plt.show() # displaying the plot