What is postpartum preeclampsia, a condition Meghan suffered from?

Duchess of Sussex Meghan Markle revealed suffering from postpartum preeclampsia and called it "rare and so scary". This condition occurs when patient has high blood pressure and excess protein in urine soon after childbirth. As per Mayo Clinic, most cases of postpartum preeclampsia develop within 48 hours of childbirth. It can cause seizures and other serious complications if left untreated.