What exactly is Python multiprocessing Module's .join() Method Doing?

Question

Learning about Python Multiprocessing (from a PMOTW article) and would love some clarification on what exactly the join() method is doing.

In an old tut

Answer 1

The join() call ensures that subsequent lines of your code are not called before all the multiprocessing processes are completed.

For example, without the join(), the following code will call restart_program() even before the processes finish, which is similar to asynchronous and is not what we want (you can try):

num_processes = 5

for i in range(num_processes):
    p = multiprocessing.Process(target=calculate_stuff, args=(i,))
    p.start()
    processes.append(p)
for p in processes:
    p.join() # call to ensure subsequent line (e.g. restart_program) 
             # is not called until all processes finish

restart_program()