Its a internet service/company that wants to make HTTPS available for everyone. Because of security reasons. You can create an Lets Encrypt Account, Register your servers DNS in any DNS service out there and then link your server to your Lets Encrypt Account. Then you can get SSL cert for your DNS Record/your server. I think it is valid for 90 days. You can renew the cert when there are 30 days left i think. So its free SSL for your webserver connection. This is a realy basic explanation but i think to understand what it does its enough ;) Hope this helps - If not, feel free to ask more :)