peach, follow on is iron-supplemented formula, and it's marketed for babies over 6 months. They don't have to have it, and you don't have to buy it unless you want to make extra profits for the manufacturers!
Your HV was trying to explain the difference between 1st and 2nd stage milks, both intended for babies from birth, but 2nd stage is more usually marketed for 'hungrier' babies, because, as she says, it takes longer for the baby's system to metabolise it.
The difference is in the protein component. 2nd stage is casein dominant (the 'curds', which is the bit cheese is made of, when you are making cheese from animal milks) and 1st stage is whey dominant.
Aptamil markets itself as closer to breastmilk, because of the type of additives in it. They have managed to market this notion very successfully to healthcare professionals. That's why you keep hearing it.
It may be true - but if it is, it is true in the sense that Potters Bar is closer to the North Pole than London.