Depending on the type of SSL certificate you purchase, your certificate authority (CA) may give you both a custom SSL certificate and an intermediate certificate. This indicates that the SSL certificate was signed by an intermediate certificate authority.
An intermediate certificate can provide an extra level of security, because it represents the root certificate without exposing the root certificate directly. The root and intermediate certificates provide a certificate chain that certifies the custom SSL certificate issued to you.
You must install the intermediate certificate with your issued SSL certificate to complete the chain of trust for your custom SSL certificate.