because no one else has explained...
the lines on the barcode represent numbers. that's how the computer sees each grouping of lines when it is scanned.
on most bar codes there are two very skinny lines grouped together and the very beginning, very middle, and very end. These aren't actually used in the barcode, but instead split the left and ride string of numbers (bars).
The two skinny lines, though again aren't read by the scanner, WOULD represent the number six. Since there are three sets of the two skinny lines (beginning, middle, end), some folks thinks that it's the mark of the beast....666
i say horseshit and think it's a matter of humans being really good at seeing patterns that aren't there, but that's just me