Cant understand callbacks lesson 9

Hi , I´m having some trouble understanding how callbacks work , I mean I do understand what they are meant for but no how they operate. Having the following “fit” function , why does it return “for epoch in range(epochs)” if every “cb.begin_fit(learn)” callback returns a “True” statement.

def fit(epochs, learn, cb):
    if not cb.begin_fit(learn): return
    for epoch in range(epochs):

class CallbackHandler():
    def __init__(self,cbs=None): = cbs if cbs else []

    def begin_fit(self, learn):
        self.learn,self.in_train = learn,True
        learn.stop = False
        res = True
        for cb in res = res and cb.begin_fit(learn)
        return res

class TestCallback(Callback):
    def begin_fit(self,learn):
        self.n_iters = 0
        return True