What is it? Well, to answer your question (from the Mayo Clinic website):
A molar pregnancy — also known as hydatidiform mole — is a noncancerous (benign) tumor that develops in the uterus. A molar pregnancy occurs when the placenta develops into an abnormal mass of cysts rather than becoming a viable pregnancy.
Molar pregnancy is a type of gestational trophoblastic disease (GTD). In a complete molar pregnancy, there's no embryo or normal placental tissue. In a partial molar pregnancy, there's an abnormal embryo and possibly some normal placental tissue. The embryo begins to develop but is malformed and can't survive.
A molar pregnancy can have serious complications — including a rare form of cancer — and requires early treatment. Careful monitoring after a molar pregnancy can help preserve the chance for future healthy pregnancies.