Client-server applications use the TLS protocol to communicate across a network in a way designed to prevent eavesdropping and tampering. TLS typically relies on a set of trusted third-party certificate authorities to establish the authenticity of certificates. Trust is usually anchored in a list of certificates distributed with user agent software. wikipedia 
Let’s Encrypt is a global Certificate Authority. We let people and organizations around the world obtain, renew, and manage TLS certificates. Our certificates can be used by websites to enable secure HTTPS connections. site