Python integer partitioning

Hello, can someone please explain me this piece of code line by line?

def gen(n):
    # tuple version
    if n == 0:
        yield ()
        return

    for p in gen(n-1):
        yield (1, ) + p
        if p and (len(p)<2 or p[1] > p[0]):
            yield (p[0] + 1, ) + p[1:]

I'm mostly troubled with the statement "yield", and while I've googled for examples, none of them seem to be helping me understand this code as none of them use it in this way.