SSL/TLS
The cryptographic protocols that encrypt data traveling between a web browser and a website, shown by the padlock icon.
SSL/TLS↗ (Secure Sockets Layer / Transport Layer Security) refers to the cryptographic protocols that encrypt data as it travels between your web browser and a website's server. TLS is the modern, more secure successor to the older SSL protocol, though the terms are still often used interchangeably, and it's what puts the padlock icon and "https" in your browser's address bar.
Without SSL/TLS, information you submit to a website, passwords, credit card numbers, personal messages, would travel across the internet in plain, readable text that anyone monitoring the connection could intercept. A valid TLS certificate also helps confirm you're actually talking to the real website rather than an impostor.