Moire patterns are those little unpredictable artifacts that sometimes pop up when small dot patterns or certain mechanical tones are employed or over-layed. Here’s an example borrowed from Hi Fi Color’s excellent website Master Digital Color –
The best way to avoid moires is to be sure that your tone or dot pattern renders are always done TO final print resolution. If you render your pages at less than final print resolution and “rez up” then your chances of generating moires are greatly increased. Similarly, over-rezzing your renders (rendering at higher than print resolution) and then re-sizing down will also likely create moires.