Implantation sac??

Jordan • Blessed.
Has anyone ever heard of an implantation sac? I'm 6w1d and went for an US today they found 2 sacs and said one is full of blood. The sac with a heart beat is measuring right on schedule and looks sold and healthy. I guess my concern is not knowing anything about it, the US tech said she has seen it a lot, I've just never heard of it!?