Terms
HTTP with encryption
Nobody sitting between you and the site can read or alter what is sent.
HTTPS is HTTP with a layer of encryption on top. Without it, anyone on the same network can read what passes between a visitor and the site, including passwords and card numbers.
With it, that traffic is scrambled. Someone intercepting it sees nothing useful.
Where you meet it
The padlock in the address bar. Browsers now mark sites without HTTPS as “not secure”, and search engines rank them lower.
Turning it on requires a certificate, which proves the site is who it claims to be. Most hosts issue one free and renew it automatically, so this is rarely something you pay for separately.
External resources