What is the AttributeOverride annotation used for Hibernate

@Entity
class User {
@EmbeddedId
@AttributeOverride(name="firstName", column=@Column(name="fld_firstname")
UserId id;
Integer age;
}
@Embeddable
class UserId implements Serializable {
String firstName;
String lastName;
}

I want to know what the use of AttributeOverride is. This is the code from hibernate interactive docs