It's a form of miscarriage where the sac and placenta grow but the baby doesn't form. The sac and placenta are what produce hcg so for most people, you get all the symptoms but have no idea until you go for a scan and the sac is empty. I never heard of them either until I had one. I went on to have an amazing and healthy little boy though so it's usually a one off. Good luck on your TTC journey!