SSL
aka: httpsaka: tlsaka: certificate
The Quick Version
The lock icon. Encryption between the browser and your site.
How it actually works
SSL/TLS encrypts traffic so nobody between the visitor and your server can read or modify it. Hosts issue certificates automatically once DNS points at them correctly.
Why does it matter?
Browsers actively warn visitors away from sites without it, and forms without it are unsafe.
Example
https:// instead of http://