Meaning of plt.subplot()

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Python

I'm studying and playing around with a piece of code and Im confused as to what the line plt.subplot(121) means. I've tried changing the 121 and it seems to make the graph wider or narrower or disappear altogether. It's used again but with 122 instead. Could anyone help me out with its meaning/function?

What I have tried:

from scipy.stats import linregress
import numpy as np
import matplotlib.pyplot as plt

data = np.loadtxt('EP305_lecture_11_data1.dat')

xcol, ycol = 0, 1
x, y = data[:,xcol], data[:,ycol]

S, y0, r, _, sigmaS = linregress(x,y)
print(f'Fitted model is y={S:0.3g} x + {y0:.3g}.')
print(f'Uncertainty of the slope is {sigmaS:.3g}.')

plt.subplot(121)
plt.xlabel('x [l]')
plt.ylabel('y [l]')
plt.plot(x,y,'kx')
plt.plot(x,S * x + y0)
plt.subplot(122)
plt.xlabel('x [l]')
plt.ylabel('residuals [l]')
plt.plot(x,y-(S * x + y0),'k')
plt.tight_layout()