If nothing on your design goes to any of the four edges or near any of the four edges, then this page can be ignored. But if a background image or photo or text block does go to or near an edge, then please read this sheet.
Bleed: Most page layout programs will allow a bleed to be set. This would normally be 2mm or 3mm and it is an area outside the trimmed page size where objects need to be put if the object goes to an edge of a page. The idea is that any variation in printing or guillotining positions will not result in a thin white line.
Image creation programs such as Photoshop need to be treated differently. Here, the canvas size needs to be made larger than the final sheet size by 2mm or 3mm. Then guidelines are drawn to indicate the page area.
Safe area: This applies to text or any other important item such as a logo. These should not be positioned closer than 3mm from the edge of the page. This is again to allow for variation in printing and guillotining.