The effect which an early association of ideas has upon the character
This chapter deals mostly with the disastrous effects that a bad early-life education can have on women. Wollstonecraft was smart enough to know that the first years of life tend to be the ones that form our character. That's why she wants to make sure that we use these years to give every child the best opportunity to develop.
Women tend to be stunted by early education because they're taught from the earliest age to be ladylike and never taught anything after that. This education keeps them in a childlike state for their entire lives. They end up only knowing things that they've memorized by rote, but they don't understand the deeper principles beneath anything.
A lack of proper early education is one of the main reasons why women are so easily seduced by "rakes" (sexually promiscuous men). All they've ever learned is to chase after compliments, so they're in heaven as soon as a man starts complimenting them.
Every day, women are seduced and "ruined" by men because these women haven't learned how to think for themselves and to recognize deception when they see it. Wollstonecraft would like for society to turn this pattern around by giving women a good education from day one.