The Let's Encrypt certificate authority provides completely free SSL/TLS certificates that are accepted by all modern browsers. This means that with a Let's Encrypt certificate, you and your site visitors will be able to access your website through a secure connection (https://) without receiving warnings. Certificates are renewed every 60 days through a process that is completely automated.
The major advantage of Let's Encrypt certificates is the free automated process for obtaining them, along with the encryption that they provide. They are domain-validated certificates, meaning that the verification consists only of confirming that they are installed by the domain owner. Unlike commercial certificates, they do not provide the green bar of organization validation certificates, a warranty of misuse or mis-issuance, or additional features such a site seal.
You can read more about Let's Encrypt on their web site: https://letsencrypt.org/.
In case your hosting provider offers the ability to install Let's Encrypt certificates, you will see a Let's Encrypt section in your hosting Control Panel > SSL/HTTPS section.