SSL used to be confusing, expensive and typically only required for web pages that were passing financial transactions and sensitive data. They were primarily only used on shopping cart pages. But as more and more web applications were being built to perform everyday tasks, Google and Firefox decided it was important to make all pages secure.
The problem was that SSL Certificates were expensive and not easy to implement until ISRG came along.
The Internet Security Research Group (ISRG) came along and decided to help make the internet a safer place. The ISRG is a 501(c)(3) whose mission is to reduce financial, technological, and educational barriers to secure communication over the Internet.
They created Let's Encrypt a free automated open Certificate Authority.
All Connecting Member's websites now have SSL at no cost to you. There's nothing you need to do. Existing websites will be updated automatically.
Here is a video that explains SSL in basic terms.