The key principles behind Let’s Encrypt are:

Free: Anyone who owns a domain can get a certificate validated for that domain at zero cost.
Automatic: The entire enrollment process for certificates occurs painlessly during the server’s native installation or configuration process, while renewal occurs automatically in the background.
Secure: Let’s Encrypt will serve as a platform for implementing modern security techniques and best practices.
Transparent: All records of certificate issuance and revocation will be available to anyone who wishes to inspect them.
Open: The automated issuance and renewal protocol will be an open standard and as much of the software as possible will be open source.
Cooperative: Much like the underlying Internet protocols themselves, Let’s Encrypt is a joint effort to benefit the entire community, beyond the control of any one organization.