CodeIgniter 3 through composer?

I just installed CodeIgniter 3 through composer by kenjis.

after installing I noticed the root folder is exposed.
and there are these /public/ and /vendor/ sub-folder.
So how do I make the root folder run the index.php?

sorry I'm confused with this new settings.

Or can someone please explain to me in simple english the purpose of /public/

thanks in advance.