[wp-hackers] two-underscore function arguments?

[Ghodmode]

What's the syntax of the __() function?  Can someone send me a link to
the documentation that explains it?

I'm looking through the code of an existing WP plugin and I hope to be
able to change it to meet my needs.  I've seen the __() function for the
first time and I'm wondering exactly how it works.  Here's an example:

__('Duration', 'wpevents')

Searching the web tells me it has something to do with translation, but
all of the examples I've seen only have one argument.  What's that
second argument?

Thank you.

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Ghodmode
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