Carotid ultrasound is done to assess the extent of blockage or narrowing of the carotid arteries.
Mild to moderate plaque means calcification of your arteries which happens with age.
Calcified arteries have less capacity to expand for increasing the blood flow when demand is increased in the brain.
In other words, moderate calcification may increase the risk of stroke.
Controling the blood pressure, cholesterol can reduce the chances of further calcification.