AutoSSL is the cPanel feature that requests and renews your certificates for you, using Let's Encrypt.
What it covers
It covers every domain, subdomain, addon domain and parked domain on your account, plus the service subdomains such as mail, cpanel and webmail, as long as each of them resolves to this server.
When it runs
AutoSSL checks your account regularly and renews any certificate that is approaching expiry, well before it runs out. You do not need to do anything, and there is no renewal to pay for.
Checking your certificate
In cPanel, open SSL/TLS Status. Every domain on the account is listed with the status of its certificate and the date it expires. If a domain shows as not covered, that screen will usually tell you why.
The usual reason a domain is not covered
AutoSSL has to prove to Let's Encrypt that the domain really points here, which it does by placing a file on the site and fetching it over the web. If the domain does not resolve to this server, or something blocks that request, validation fails and no certificate is issued. See Why my SSL certificate did not renew for the specific causes.
