This is often a Schedule or Annex to the actual agreement. It is the certificate your solicitor will sign to confirm that they have given you advice as to the terms and effect of the agreement on your ability to pursue any claims you have.
The certificate also confirms the status of your legal adviser as they must meet the strict legal definition to be able to provide advice. Often this will confirm that they are a solicitor with a practising certificate and finally that the firm has a certificate of insurance covering the risk of a claim by you for negligent advice.
In order to be a binding agreement the solicitor will need to sign and return the advisers certificate to your employer.