I think it means for internal fertilisation, more energy and effort from parents = very few gametes / and potential offspring
for external fertilisation, less energy and effort from parents = many gametes
so in both cases there's indirect or inverse proportionality between the investment from parents during copulation, and the offspring that could come out of it, like if one quantity is big the other quantity is small and vice versa. honestly I think this point is worded really strangely I wouldn't bother dwelling on it