[web2py] get_content

[pftpft]

I'm new to web2py.  I've scanned thru most of the "book" and have set
up web2py on our local apache server.  Inside of the 'examples'
application, I pulled up the default/index.html view and see the line:
    {{=get_content('maincontent')}}
I've done various searches and don't see where get_content is
explained.  It looks like it is pulling in from the private/content/en/
default/index folder, but why is not obvious.  Where is this covered?
Thanks.