The edge server to origin connection
For this connection in an HTTPS request, the origin server must also have a certificate (the "origin certificate") as well as a private key for your site that are trusted by the Akamai Edge server. (With this connection, the Edge server is serving as the client.)
By default, the Edge server performs a check similar to what a browser does in the first connection: it checks that the certificate was signed by a trusted Certificate Authority, and that the cert is legitimate for the requested site.
Optionally, you can configure the Edge server to trust any certificate or Certificate Authority you want. You can learn how to obtain and configure this certificate on your origin server, and what custom settings you can configure for the origin certificate check, in Create an origin certificate.