You use she to refer to a woman, girl, or female animal who has already been mentioned or whose identity is clear.

Some writers may use she to refer to a person who is not identified as either male or female. They do this because they wish to avoid using the pronoun 'he' all the time. Some people dislike this use and prefer to use 'he or she' or 'they'.

She is sometimes used to refer to a country or nation.

Some people use she to refer to a car or a machine. People who sail often use she to refer to a ship or boat.