Servers and hosting
TLS and HTTPS certificates
The encryption behind https:// and secure mail delivery, and the certificate that proves a server is who it says it is.
TLS encrypts the connection between a client and a server and lets the client check the server's identity. HTTPS is HTTP over TLS; IMAPS, SMTPS and the STARTTLS variants are the same idea for mail.
The identity check rests on a certificate: a public key for the server's name, signed by a certificate authority the client trusts. Browsers and operating systems ship a list of trusted CAs; a certificate from one of them for the right name, within its validity dates, is accepted. Let's Encrypt made them free and automated, so there is no reason to serve anything without one.
Certificates expire, which is why automation matters: a web server or panel that renews them itself every few weeks never has an outage from an expired certificate. CAA records control which CAs may issue for your domain, and DANE lets mail servers verify your certificate through DNS without trusting any CA at all.