I'm not a professional. My understanding of lacunae derives entirely from one doctors explanation of what happened in my case (which did involve placenta previa). I could be wrong.
My understanding is that there are, in general, in a healthy placenta, pools of blood called lacunae.
In some cases (previa being the one that, naturally, springs to my mind, but also in cases of abruption, partial abruption, or other issues), the lacunae can rupture. If the particular lacuna is small, the bleeding may be inconsequential. If it's big, the blood loss can be dangerous for mother and baby. Even small bleeding causes some concern as the lacunae to some extent protect the oxygen transfer membrane between mother and babe, and damage to that membrane can result in fetal oxygen deprivation (which can result in severe brain injury or in death).
It's possible that the woman in your CB class didn't entirely understand her doctor's explanation. Frankly, the reason I understood my doctor's explanation was that I asked for it six weeks after the fact. Much of what was said to me about risks in my pregnancy went right over my head, while I frantically tried to process the idea that there were risks in my pregnancy.