The meaning is very simple, which can be understood as like son, like father, like father, like son. It can also be understood in another way. Superman sighs that when the son becomes the father (Superman himself becomes the father here), the father also becomes the son (and he is also the father's son). See how to understand, I personally prefer the latter. The former is the original intention, while the latter has the emotional scene of Superman seeing his family and missing his father.