What exactly is a matplotlib axes object?

When we create a new figure:

import matplotlib.pyplot as plt

fig = plt.figure()

we can see in the console an output like:

<Figure size 432x288 with 0 Axes>

so when we use add_axes:

add_axes(rect, projection=None, polar=False, **kwargs)

are we actually defining the x,y axes that encompass the "box" that will bound the figure (the axes in a more mathematical sense) and nothing more? or in fact this line of code is creating an empty figure with the desired dimensions in which any data we add later will be fitted in? (or None of the above maybe?)

That questioning left me wondering how can I physically understand what the axes are for matplotlib.

Thanks for helping.