We've closed quite a few questions as "off-topic". This close reason references the Help Center's on-topic page. This page is currently near-empty, and provides no information as to what is on-topic and what is not. We have many questions on meta concerning the scope of the site, but these aren't referenced, linked to or mentioned in the Help Center.
This post is to discuss what information should be put on the on-topic page. Please provide one topic per answer, preferably with a suggested wording.