How an SSL certificate works
A certificate is a small file issued by a certificate authority and installed on your server. It ties your domain name to a cryptographic key, and it lets the browser and the server agree on a scrambled connection before any page content is sent. Anyone sitting on the network in between — a shared office router, a café hotspot, a mobile operator — can see that a connection to your domain exists, but not the passwords, form fields or card details travelling inside it.
The certificate does two separate jobs. It enables the encryption, and it vouches for identity: it is proof that the server answering for your domain is the one entitled to. Certificates are issued for a fixed term and then expire, which is why renewal is the part that bites. Most hosts now issue and renew one automatically, and the encryption they provide is the same as a paid certificate; paid options add checks on the organisation behind the domain rather than stronger protection.
Why HTTPS matters
Browsers mark plain HTTP pages as not secure, and they do it most loudly on any page containing a form. A visitor who has never thought about certificates still reads that label as a reason to leave, and payment gateways will not process transactions over an unencrypted connection at all.
There is a measurement argument too. Traffic arriving from a secure site to an insecure one loses its referrer, so the source disappears into direct traffic and your reporting quietly degrades. Search engines have long treated HTTPS as a positive, if minor, signal.
Where SSL goes wrong
Expiry is the classic failure. Nothing about the site is broken, yet every visitor meets a full-page warning that most will not click past, and search engines can fail to fetch the pages while it stands. It is the most avoidable outage there is.
Next is mixed content: the page itself loads securely but an image, script or stylesheet is still requested over plain HTTP, so the padlock disappears or the asset is blocked outright. After that come coverage gaps — a certificate issued for the www address but not the bare domain, or a subdomain nobody remembered — and the redirect that was never set, leaving both an HTTP and an HTTPS copy of every page reachable at once. That duplication is worth fixing with a proper permanent redirect rather than a temporary one.
How to act on it
Automate the renewal, then monitor the expiry date independently, because automation fails quietly. Force one canonical secure address and redirect everything else to it in a single hop. After any switch, crawl the site for internal links and hardcoded asset paths still pointing at HTTP, and check that your analytics property, Search Console property and ad destination URLs all use the secure version.
Treat the certificate as one item on a maintenance rota rather than a launch task. Domain renewal, certificate renewal and server updates all cause outages that look identical from the outside, and the same routine website maintenance schedule should cover all three.