In Matplotlib, what does the arg

It is always the best way to explain the complex idea as simple as we go along with some practices or illustration >.<

import matplotlib.pyplot as plt
fig = plt.figure()
fig.add_subplot(221)   #top left
fig.add_subplot(222)   #top right
fig.add_subplot(223)   #bottom left
fig.add_subplot(224)   #bottom right 
plt.show()