When the stripes on your guys,
make a swirly surprise,
That’s a moire….
When the stripes on your girls,
turn into little curls,
That’s a moire….
But seriously, if you don’t know what a moire pattern is you can check out a decent explanation at wikipedia

How to fix them,  the best method that I have seen is to resize the image in photoshop.  To do this you go to the menu at the top of the screen. Select Image, then go down the list and select “image size”.  A dialogue box will open.  Make sure the “resample image” box is checked, then change the number in the resolution box to a slightly smaller number. Finally, don’t save.  Instead select “save as” and add version1 or some other add on to the end of the file name.  This way you can keep changing from the original file until you find a resolution that eliminates or greatly reduces the moire pattern in your image.

here are a couple of other great resources for dealing with moire patterns: