I haven't had it, but a friend of mine was thought to have it.
Basically it means the placenta grows right into the wall of the uterus and so it can be very difficult for it to come out, or be removed safely. For my friend it meant having lots of scans through her pregnancy and the plan was to have a c section, and they try and remove the placenta, in the worst case scenario if it is impossible to remove and the mother is losing lots of blood they would have to have a hysterectomy to save their life. My friend was very worried about this possibility.
I think at times it can be hard to get a good idea exactly what is going on with the scans, and it can change over time. Anyway, she had lots and lots of scans and I think an MRI. Near the end of the pregnancy it was found that it wasn't grown through the wall of the uterus afterall and she was able to have a normal birth! It doesn't affect the baby usually.
Have they talked to you about any of the possibilities? Are you having more scans?