The Intermediate Certificate acts as a middle-man between the Certificate Authority and the end-user SSL.
A Root CA is a Certificate Authority (in this case, Digicert) that owns one or more trusted roots.
That means that they have roots in the trust stores of the major browsers.
An intermediate CA is a Certificate Authority that issues off an intermediate root.
They do not have roots in the browser’s trust lists, instead their intermediate roots chain back to a trusted third-party root (the root from Digicert).
The intermediate certificates are included in the “Pem Chain” download.
You can also download the Intermediate Certificates and Root Certificate separately from Digicert here:
https://www.digicert.com/kb/digicert-root-certificates.htm
More Information
You can find more information about Digicert SSLs from their Knowledge Base.