Off: no SSL between the visitor and CloudFlare, and no SSL between CloudFlare and your web server (all HTTP traffic).
Flexible: SSL between the visitor and CloudFlare -- visitor sees HTTPS on your site, but no SSL between CloudFlare and your web server. You don't need to have an SSL cert on your web server, but your vistors will still see the site as being HTTPS enabled.
Full: SSL between the visitor and CloudFlare -- visitor sees HTTPS on your site, and SSL between CloudFlare and your web server. You'll need to have your own SSL cert or self-signed cert at the very least.
Full (Strict): SSL between the visitor and CloudFlare -- visitor sees HTTPS on your site, and SSL between CloudFlare and your web server. You'll need to have a valid SSL certificate installed on your web server. This certificate must be signed by a certificate authority, have an expiration date in the future, and respond for the request domain name (hostname).
(https://support.cloudflare.com/hc/en-us/articles/200170416)
