In this context it means moving (rhetorically) in a different direction from the main subject.
Proper usage of the term requires the secondary or "tangential" subject to be related in some way (even if only tenuously) to the primary subject, otherwise it would be a non-sequitur.
Max and JTT have been off on wild
tangents coming very close to non-sequitur.
A more apt tangential departure from the original subject would be the comment made about Bill Clinton's Native American heritage.
Comments made about Obama's African heritage and where he was born may be tangential
to the comment about Clinton, but are non-sequitur
as regards the original discussion.
As you probably have noticed, A2K members go off on tangents all the time, and so the threads often appear as rivers with many branching