Explain cell count and embryo grading?

Melissa • TTC 3 years. IVF BFP 9/17. 🥚due 5/18

I had honestly never heard of the cell count or grading for embryos before our transfer. I didn't ask for details the day of mostly because I was on Valium and too sleepy to care but now I'm curious. What does all that mean? The doctor said our embryos were great quality but I don't know much more than what I have googled. How did your clinic explain it to you? These are our embryos.