Bleeds are the extra printed image that extends beyond where the page is going to be trimmed. If you have an image or a color field you want to run all the way to the edge of the page, you’ll need to do a bleed. Remember that when measuring a bleed, the measurement is for each side of the item.
For example, a 1/8 inch bleed extends 1/8 inch beyond the crop marks on each side. This means that the finished product will be a total of 1/4 inch narrower horizontally and vertically than the image printed upon it, once the item is trimmed.
If a bleed is not used, even the slightest misalignment in the printing, cutting, or original image will result in a tiny amount of the image being lost, or a sliver of white paper showing at the very edge. Think of bleeds as an insurance policy against such things, and you will have a more professional and polished final piece.
For further details, we recommend that you read our file preparation guide.