ACME(Automatic Certificate Management Environment)
The protocol used to automatically obtain and renew free TLS certificates, most famously from Let's Encrypt.
Tools like Caddy, Traefik, Nginx Proxy Manager and Certbot speak ACME to prove you control a domain (via HTTP or DNS challenges) and then issue certificates without manual work. This is how homelab services get automatic HTTPS.
Related articles
TLS/SSL
The encryption that secures connections, shown as the padlock and https:// in your browser.
Let's Encrypt
A free, automated certificate authority that issues the TLS certificates used to enable HTTPS.
Reverse Proxy
A server that sits in front of your services, routing requests by domain name and handling TLS certificates.