In Cocoa apps (e.g., most of the default OS X apps except Finder, iTunes, etc.),
pressing Help would turn the mouse into a question mark.

But clicking on anything doesn't have any effect.

I'm guessing it's meant to describe or provide help on whatever you're clicking on?
(if that's the case, then pretty much NO app out there actually does it, not even Apple's own apps)

side note:

In Carbon apps, it has different effects.

For instance:
In the Finder, it doesn't do anything.
In iTunes, it brings up iTunes Help in the Help Viewer.