SSL and HTTPS
HTTPS means the connection between a visitor and your site is locked, so nobody on the way can read or change it.
In a simple example
A site with a small padlock and an address that starts with https:// is using it. Browsers warn people on sites without it.
Why it matters to you
Visitors trust a locked site, and browsers show a warning on sites that are not locked.
Do not confuse with
SSL is the older name of the lock; today it is really called TLS. People still say SSL.