The -- operator is the auto-decrement operator and is a common construct in modern programming languages, including C, PHP, VBScript, Java and JavaScript and Perl. Basically its effect is to reduce the numerical value of a variable by one. It has a different effect depending on how it is used:

$y = 5;
$x = $y--;
# $y is 4, but $x is 5.
compare to the following:

$y = 5;
$x = --$y;
# now both $x and $y are 4.
Using the -- operator after the variable causes the decrement to occur after the value is returned. Using the operator before has the reverse effect.

The -- operator is the obverse of the ++ operator, which was inspired by the "newspeak" in 1984. Like ++, -- has slipped into hacker and geek vocabulary to indicate a perceived decrease in the value of the subject.

See also ++ and the nodes on Perl, C, Java and PHP and the perlop manpage.

This has been a nodeshell rescue. Thanks to ariels for correcting a moderately bad braino on my part.