What’s the ‘+’ in a chord symbol about?

Someone asked this on one of the forums recently.

The answer is (in the generally accepted though not-at-all-internationally-standardized way) that it means the chord is an ‘Augmented’ chord, i.e. it has a raised (sharpened) 5th.

C major chord spelling: C E G

C Augmented (C+) spelling: C E G#

Try playing a C+ chord (or variants), then whack some notes of the associated augmented/whole tone scale over it. It has a floating ‘non-resolving’ feeling.