SSL (Secure Sockets Layer) is what adds the HTTPS in a web address and the padlock next to it.
SSL encrypts data submitted through your website. Whether it's payment details or contact information, this data is encrypted on its journey from the user's browser to the web server.
Prior to 2018, SSL was only required for websites that took payment details. Standards have changed and it should now be present on any and all websites.
If you haven't adopted SSL for your website yet, it's worth reading into why it's a must have.