When a customer has a Connector rule for an HTTP-based protocol such as HTTP, SOAP, REST, and sometimes Email, they may point to an SSL-enabled ("https") endpoint. The service that is connected to will provide an SSL certificate in order to identify itself and secure the connection.
Pega Platform relies on the Application Server in order to "trust" the certificate that another service provided. When Pega Platform is deployed in Tomcat, this usually means that the default java truststore is in use. IBM WebSphere has its own truststore, controlled in the Admin Console.
When the certificate provided by a service is not in the truststore, or otherwise not trusted (for instance, it is out of date or issued to a different organization), Pega Platform cannot complete the connection and an exception such as "Peer not authenticated" results.
It is the responsibility of the customer to ensure that the application server's truststore is set up correctly.