SSL (Secure Sockets Layer) is a standard security technology for establishing an encrypted link between a server and a client
Salesforce acting as the client
When sending outbound messages, delegated authentication requests or Apex callouts to secure/SSL endpoints (e.g. https://myintegration.acme.com), a Salesforce.com organization (acting as the client) will only trust the target host (that will act as the server) if this presents a certificate signed by a root Certification Authority (CA)
Salesforce acting as the server
When a Salesforce organization is Single Sign On enabled using SAML, the organization plays the role of the Service Provider (SP). In this case Salesforce acts as the server and the configured Identity Provider (IdP) acts as the client, and it's allowed to present a self-signed certificate.
Refer to the H&T portal for more details.