Certificates mean something only because of who issued them.
WHAT A CERTIFICATE AUTHORITY IS
An organisation that verifies domain control, or identity, and issues certificates asserting it. Browsers and operating systems ship with a list of authorities they trust.
WHY THE LIST MATTERS
If any authority could issue a certificate for any domain with no oversight, certificates would prove nothing. Authorities are audited and can be removed from browsers' trust lists if they misbehave, which has happened.
WHAT THIS MEANS FOR YOU
A certificate from a trusted authority works everywhere without warnings. A self-signed certificate, or one from an untrusted source, produces a warning on every visit.
WHICH AUTHORITY ISSUES AUTOSSL CERTIFICATES
cPanel's AutoSSL uses a trusted authority, which is why the certificates work identically to paid ones in every browser.
SELF-SIGNED CERTIFICATES
You can generate one yourself. It encrypts the connection but nothing vouches for it, so browsers warn. Acceptable for internal testing, never for a public site.
