Have you ever used proxy/VPN? It’s same method with Cloudlare.

If you use Cloudflare & turn on its proxy for your (sub)domain (orange icon),this is your visitor route to your site:
Your visitor browser → Cloudflare server → Your hosting server
Maybe you can call it “transit”? 
This is your visitor route without Cloudflare (direct route):
Your visitor browser → Your hosting server
For security,your visitor route must use SSL.
But,what if you only activate it on your hosting server?
Direct route:
Your visitor browser -[SSL]-> Your hosting server
Cloudflare route:
Your visitor browser -[No SSL]-> Cloudflare server -[SSL]-> Your hosting server
So,it’s up to you.
If you confused about Cloudflare SSL settings:
Hey,Cloudflare. I don’t have SSL for my hosting server. I just want to use your SSL.
Your visitor route:
Your visitor browser -[SSL]-> Cloudflare server -[No SSL]-> Your hosting server = PASS
Your visitor browser -[SSL]-> Cloudflare server -[SSL (expired/not valid/valid)]-> Your hosting server = PASS
Hey,Cloudflare. I have a SSL certificate,but it isn’t valid (self-signed certificate).
So,don’t let my visitor pass to my site if my certificate expired or I forgot to install it on my hosting server.
Your visitor route:
Your visitor browser -[SSL]-> Cloudflare server -[No SSL/SSL (expired)]-> Your hosting server = NOT PASS/show Cloudflare error page
Your visitor browser -[SSL]-> Cloudflare server -[SSL (not valid/valid)]-> Your hosting server = PASS
Hey,Cloudflare. I have a valid SSL certificate.
So,don’t let my visitor pass to my site if my certificate not valid,expired,or I forgot to install it on my hosting server.
Your visitor route:
Your visitor browser -[SSL]-> Cloudflare server -[No SSL/SSL (expired/not valid)]-> Your hosting server = NOT PASS
Your visitor browser -[SSL]-> Cloudflare server -[SSL (valid)]-> Your hosting server = PASS