Fertility windows

Hayley • Just an Auzzie girl with a big dream!
What is the deal with fertility windows because I am super confused. Ok, you know on the period wheel on the app where it's purple before and after period then in green it says "fertility window". I'm so confused. Does that mean that in the green I can get pregnant and purple means I can't? Or other way round? Explain please because google ain't helpin, 😱