Not sure what your friend was meanign either.
The first milk is the colostrum
Then the 'breastmilk comes in' and as far as I thought here it stays until you stop. BUT breastmilk is extremely clever stuff, it changes as your baby gets older in order to sustain the baby as it grows. If a baby was born at say 30 weeks, the milk produced would be appropriate to the needs of a preemie, the milk I make for Andrew now at age1 will be different to the milk I made 11 months ago.
Alex