Let’s Encrypt is a free, automated, and open Certificate Authority. It uses ACME protocol to validate your domain. If you have complete control over your domain, you can get a certificate for free. In order to provide secure access to your public network like HTTPS, LDAPS etc you need a certificate from a Certificate Authority. The cost of the certificate range from 10$ to 100$. If you want a wildcard certificate then it may cost more. The certificate is valid for one year and you need to pay and renew every year. Let's Encrypt comes for the rescue. You can create and renew certificate for few.