TLS encrypts the connection between a visitor's browser and the website server. HTTPS protects credentials, form submissions and other data from being read or modified easily while it travels across the network.
What to consider
An SSL certificate does not make the application itself secure. Vulnerable plugins, weak passwords, outdated software and unsafe server configuration can still be exploited even when the browser shows a padlock.
Use HTTPS everywhere, automate certificate renewal where possible and combine TLS with application updates, access controls, monitoring and reliable backups.