Should I avoid breastfeeding in front of my 6 yr old daughter? How should I explain breastfeeding to her?

No, no, no! No, absolutely not. You should not avoid breastfeeding in front of anyone, especially your daughter. You explain to her that the reason women have breasts is to feed their babies when they are born. Your daughter will have a positive role model and learn at an early age the purpose of her body.
No. You do not have to avoid breastfeeding in front of your daughter. In fact, in many cultures breastfeeding is much more public than in the US and daughters learn from an early age how to breastfeed babies and have an expectation that they too someday will do this. You can explain how breastfeeding is a healthy way to feed the baby and then honestly answer any questions she may have.