The Let's Encrypt certificate authority provides completely free TLS certificates that are accepted by all major browsers. This means that with a Let's Encrypt certificate, you and your site visitors will be able to access your web site through a secure channel (https://) without receiving warnings. Certificates are renewed every 60 days, through a process that can be 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 at your hosting Control Panel -> section SSL/HTTPS.