An ascribed role is a social identity or title that is given to a person based on factors they have no influence over like gender, age, or ethnicity. This is a role given to us by others with no input from ourselves.
For example, in the US a person is considered a teenager when they are ages 13-18. A person who is from Europe is called a European. The opposite of this is an achieved role which is one that an individual has chosen or earned.