Oneboxing is a fancy word for inlining content from another site. The syntax to use it is simply to paste the raw URL, and only the URL, on a line by itself. The sites which will be so 'onebox'ed are contained within a whitelist. Don't complain if your link to
won't get oneboxed like a link to, say twitter
What the contents of the onebox will actually be appears to be dependent on the source - Wikipedia will have a snippet, Twitter seems to omit pictures as shown, YouTube will have the whole video window visible and will play in-situ.
Note that the URL must be publicly accessible - i.e. without having to log in. For example:
http://what.thedailywtf.com/t/level-progression-requires-action-from-user/1175 (requires being logged in and being a level 1 user) produces
i.e. no differnce.