How certificate chains work
A certificate chain is an ordered list of certificates, containing an SSL Certificate and Certificate Authority (CA) Certificates, that enable the receiver to verify that the sender and all CA's are trustworthy. The chain or path begins with the SSL certificate, and each certificate in the chain is signed by the entity identified by the next certificate in the chain.
Any certificate that sits between the SSL Certificate and the Root Certificate is called a chain or Intermediate Certificate. The Intermediate Certificate is the signer/issuer of the SSL Certificate. The Root CA Certificate is the signer/issuer of the Intermediate Certificate. If the Intermediate Certificate is not installed on the server (where the SSL certificate is installed) it may prevent some browsers, mobile devices, applications, etc. from trusting the SSL certificate. In order to make the SSL certificate compatible with all clients, it is necessary that the Intermediate Certificate be installed.
The chain terminates with a Root CA Certificate. The Root CA Certificate is always signed by the CA itself. The signatures of all certificates in the chain must be verified up to the Root CA Certificate.
The below figure illustrates a certification path from the certificate owner to the Root CA, where the chain of trust begins: