What is Let’s Encrypt? Let’s Encrypt is a Certificate Authority (CA). A certificate from a CA is required to enable HTTPS.
Certbot’s documentation summarises it well:
Certbot is part of EFF’s effort to encrypt the entire Internet. Secure communication over the Web relies on HTTPS, which requires the use of a digital certificate that lets browsers verify the identity of web servers (e.g., is that really google.com?). Web servers obtain their certificates from trusted third parties called certificate authorities (CAs).