Root certificate identifies the Root Certification Authority. Certification Authority issues multiple certificates in the form of a tree structure. A root certificate is the top-most certificate of the tree. All certificates below the root certificate inherit its trustworthiness (a signature by a root certificate is similar to ‘notarizing’ an identity in the physical world). A certificate signed by a Root CA is implicitly trusted by most web browsers. Intermediate certificate is the secondary certificate of CA's tree structure.

Root and intermediate (if available) certificates have to be installed on web server of the hosting server where your domain is hosted for your SSL certificate to work properly.
Comodo (now Sectigo) root and intermediate certificates are attached to email you receive from the Certificate Authority when your certificate is issued. If you have lost your email please contact our SSL Support Team via live chat or ticket and we will re-send it to you.