Placenta accreta is a condition in which the placenta grows into the uterine wall (the muscle instead of just the uterine lining). It is rare in the general population, but women who have had a c-section or other uterine surgery previously are at higher risk for it because of the scar tissue present. It is actually becoming more common as more women are having elective c-sections, often multiple times. Every subsequent c-section or surgery will increase that risk.
The most dangerous complication is that the placenta does not separate properly after birth and causes excessive bloodloss. If this happens many surgeons will perform an emergency hysterectomy to save the life of the mother.
I would strongly advise that you go over the emergency protocol with your OB/GYN BEFORE you have your c-section. Ask what the protocol is, ask what measure will be taken if you experience excessive bleeding BEFORE it has to come to a hysterectomy, ask what kind of other procedures YOUR surgeon is proficient in that may save your uterus - especially if more children are in your future plans.
GET A SECOND OPINION! Seriously talk to at least one other surgeon at a different hospital about how they would go about treating possible complications.
The only way you will be cleared is if the placenta moves completely away from your previous scar - which can happen as your uterus grows and stretches. If the placenta remains close to the scar, they will treat you as if you had placenta accreta - even if it is unsure if you actually do have it. Mostly that means having additional blood available at the time of your surgery and having everyone on standby for possible complications.
Hope everything works out for you. Good luck!