yield from

[akbarza]

hi
in the below code, what is doing yield from? please explain:

#yield_from.py
# from:https://treyhunner.com/2018/04/keyword-arguments-in-python/


def join(*iterables, joiner):
    if not iterables:
        return
    yield from iterables[0]     #? yield from=?
    for iterable in iterables[1:]:
        yield joiner
        yield from iterable

'''That joiner variable doesn’t have a default value, so it must be specified:'''
list1=list(join([1, 2, 3], [4, 5], [6, 7], joiner=0))
print(list1)    # out: [1, 2, 3, 0, 4, 5, 0, 6, 7]
list2=list(join([1, 2, 3], [4, 5], [6, 7], joiner='-'))
print(list2)    #out: [1, 2, 3, '-', 4, 5, '-', 6, 7]
# the list(join([1, 2, 3], [4, 5], [6, 7])) causes erorr.with below message:
#TypeError: join() missing 1 required keyword-only argument: 'joiner'

thanks