What is fig, ax = plt.subplots()

In Python, matplotlib.pyplot.subplots creates a figure and a set of subplots.

Default parameters for subplots are nrows=1 and ncols=1:

matplotlib.pyplot.subplots(nrows=1, ncols=1)

fig, ax = plt.subplots() create a figure with a single subplot. But the previous returns a tuple containing the figure (fig) and axes objects (ax) which us useful for future use (changing figure-level attributes, save the figure as an image file, displaying grid, axis label, title...) as on the following example:

import matplotlib.pyplot as plt
fig, ax = plt.subplots()
ax.plot(x, y)
ax.set_title('My awesome title')