IDRBTCA's root certificate do not come pre-installed in your browser. You will need it to validate a certificate which is in a certification chain with the IDRBT CA's root certificate when you encrypt or digitally sign S/MIME messages with such a certificate or when you make https connections with a IDRBT CA certified Web site.
IDRBT CA's public key is signed with the key of Controller of Certifying Authorities, Government of India, which comes as the Root Certificate of IDRBT CA, thus providing the certificate chain.
You must install the root certificates of CCA and IDRBT CA to validate the certificate chain. Download the files to a folder and install the certificates to the browser store.