Recently there was no way to get a valid free SSL and you were force to buy one .
Thanks to them you can now get a free SSL which is valid in major browsers.
The objective of Let’s Encrypt and the ACME protocol is to make it possible to set up an HTTPS server and have it automatically obtain a browser-trusted certificate, without any human intervention. This is accomplished by running a certificate management agent on the web server.
To understand how the technology works, let’s walk through the process of setting up
https://example.com/ with a certificate management agent that supports Let’s Encrypt.
There are two steps to this process. First, the agent proves to the CA that the web server controls a domain. Then, the agent can request, renew, and revoke certificates for that domain.