If your artwork needs to be printed to the very edge of the finished size document, any images, colour etc. needs to "bleed" off the edge of the paper by at least 3mm - this bleed will be printed but then cut off in the final cropping process.
3mm area outside the finished paper size. The images should project into this. This will be trimmed off.
This is the final edge of the artwork after the bleed area has been trimmed off (e.g A4)
Keep your text at least 5mm inside the trim edge to ensure that it is not trimmed off with the bleed area.