Basically: It’s dangerously high blood pressure during pregnancy – that can be fatal to both mother and child if untreated.
Symptoms: Swelling due to increased water retention (cankles, fat fingers, etc.), protein in your urine, diminishing vision and headaches.
The cause: During pregnancy your blood volume increases by 50% to support your growing child, their amniotic sac and amazing placenta, which means your heart must work twice as hard as it normally does. A weak heart will do a weak job.
In order for the placenta to attach properly, your uterus employs a network of connecting roots (blood veins) that feed the placenta your blood, which it filters and feeds your child via the umbilical cord.
If your heart is incapable of effectively vascularizing the placenta, attachment to your uterus is compromised and your blood pressure increases in an effort to compensate - as indicated by the proteins forced out of your veins by too-high blood pressure, spilling into your bladder and showing up in your urine when you test for it.
The cure: Preeclampsia is only cured by giving birth... that's why a healthy heart is the single best defense against preeclampsia.