Let's Encrypt is a free, automated, and open certificate authority made by the Internet Security Research Group (ISRG). It is sponsored by well-known organisations such as Mozilla, Cisco or Google Chrome. All modern browsers are compatible and trust Let's Encrypt.
All certificates are free (even wildcard certificates)! For security reasons, the certificates expire pretty fast (after 90 days). For this reason, it is recommended to install an ACME client, which will handle automatic certificate renewal.
There are many clients you can use to install a Let's Encrypt certificate:
Let’s Encrypt uses the ACME protocol to verify that you control a given domain name and to issue you a certificate. To get a Let’s Encrypt certificate, you’ll need to choose a piece of ACME client software to use. - https://letsencrypt.org/docs/client-options/