P. Close and P. Join in Multiporcessing. Pool

I am following an instruction from youtube to learn multiprocessing

from multiprocessing import Pool
import subprocess
import time

def f(n):
    sum = 0
    for x in range(1000):
        sum += x*x
    return sum

if __name__ == "__main__":

    t1 = time.time()
    p = Pool()
    result = p.map(f, range(10000))
    p.close()
    p.join()
    print("Pool took: ", time.time()-t1)

I am puzzled about p.close() and p.join()

when processes were closed, they did not exist any more, how could manipulate .join to them?