SSL

SSL, and its modern successor TLS, is the technology that encrypts the traffic flowing between a web browser and a web server. It is signified by the "s" in "https" or the padlock in the address bar, both of which signify that your data, like a password or a credit card number, cannot be easily read by anyone listening in between.

These days SSL is not optional. Browsers warn visitors away from sites without it, and search engines favor sites that have it. Setting up and renewing the certificate that makes it work is a standard part of planning for hosting.