Please explain what it means in laymens terms when someone dies of "fibromuscular dysplasia of the small coronary artery"?

Rare disorder: There is blockage in the heart's arteries which is not due to garden variety "hardening of the arteries" (atherosclerosis) but, instead it's due to scarring - the cause of which is not understood. If enough small arteries are involved, the result can be heart failure and/or rhythm disturbance resulting in death. My condolences for your loss.