The certificate of sponsorship is being given to the person by the employer through whom the employment is being given to the person. In simple words, the employer is responsible to provide the certificate of the sponsorship to the person.
This particular certificate of sponsorship is considered to be a very important as well as a necessary document, which is to be presented to the authorities while arriving in the U.K. it should be genuine and legal as well as it is mentioned that the person is liable enough.