Still born at 35.5 weeks. Doctor said it's due to a weak spot in the placenta and placenta calcification. What does this mean?

Hypothetical cause. Really sorry for your loss and regret that explanations offered to you are rather vague and quite hypothetical. "A week spot in the placenta" is not meaningful and "placental calcifications" which are not uncommon, are unlikely to cause a stillbirth. Unfortunately, most stillbirths and pregnancy losses are unexplained (even after careful investigations) and the list of real causes is long.