SSL certificate
Short definition
An SSL certificate encrypts the traffic between browser and server and shows up as the padlock icon in the address bar.
When a visitor fills in a form or pays online, SSL means that information reaches the server encrypted, so anyone intercepting it along the way cannot read it. The padlock and https:// in the address bar are the visible sign of that encryption.
Certificates come in different types, single domain, wildcard, or multi domain, and are usually valid for a year or renewed automatically on shorter cycles. An expired certificate triggers a warning screen in the browser.
SSL also verifies the site's identity to some degree, though it says nothing about the accuracy of its content, it works best alongside security headers and general server hardening.
Once a certificate is installed, if a single image, script or plugin on the page is still loaded over http:// rather than https://, the browser raises a mixed content warning and the padlock appears broken or missing entirely. This is the most common mistake, the fix is simply making sure every resource on the page loads over https://.
Why it matters
An unencrypted site greets visitors with a not secure warning and breaks trust instantly. For any site collecting payment or personal data, SSL is both a legal requirement and a commercial one.
Illustrative example
A workwear manufacturer collecting details on a bulk order form missed an expired certificate, form abandonment crept up for a few days before anyone noticed.
