ArcGIS Server comes configured with a self-signed certificate, which allows you to do initial testing of the ArcGIS Server site and helps you quickly verify that your installation was successful. You must request a certificate from a trusted certificate authority (CA) and configure the ArcGIS Serversite to use it. This could be a domain certificate issued by your organization or a CA-signed certificate.
Like ArcGIS Server, Portal for ArcGIS also comes with a configured self-signed certificate. If you'll be federating an ArcGIS Server site with an ArcGIS Enterprise portal, you should request a certificate from a trusted CA and configure the portal to use it.