Short answer - Full means that the connection between us and your origin has a but we do not verify the authenticity of that . Full “Strict” means that the has to be signed by a certificate authority. Longer answer is below.
Full SSL: secure connection between your visitor and Cloudflare, and secure connection (but not authenticated) between Cloudflare and your web server. You will need to have your server configured to answer HTTPS connections, with a self-signed certificate at least. The authenticity of the certificate is not verified: from Cloudflare’s point of view (when we connect to your origin webserver), it’s the equivalent of bypassing this error message. But as long as the address of your origin webserver is correct in your DNS settings, you know that we’re connecting to your webserver, and not someone else’s.
Full SSL (Strict): secure connection between the visitor and Cloudflare, and secure and authenticated connection between Cloudflare and your web server. You will need to have your server configured to answer HTTPS connections, with a valid SSL certificate. This certificate must be signed by a certificate authority, have an expiration date in the future, and respond for the request domain name (hostname).