Safely defrosting frozen food so its outer layers do not sit in the temperature danger zone while the centre thaws.
Thawing in the refrigerator is the safest general method, because it keeps the food cold and out of the temperature danger zone while it defrosts.
This is discouraged, because the outer surface can sit in the temperature danger zone between 5 and 60 degrees Celsius for too long while the centre is still frozen.
Thawed food should not be refrozen unless it has been cooked in between. Repeated freezing and thawing increases food safety risk and reduces quality.
During thawing the surface of the food can warm into the temperature danger zone and allow bacteria to grow, even while the centre remains frozen.
Yes, if the food is cooked immediately afterwards, because microwave thawing can leave warm spots where bacteria could grow if the food is left standing.