The employer costs include mandatory contractual benefits, taxes, and any other fees the local government may require from the employer to pay for when hiring an employee in a particular country.
The total employer cost will vary according to the employment country, specific state or province, and the employee salary. For example, in Canada, the employer cost will cover the Canada Pension Plan, Employment Health Tax, Parental Health Benefit, and Workers Compensation Insurance.
For details on the quote process, which is used to determine employer costs, check out this article.