Bleed indicates the extra mm that graphics, color, etc. should extend beyond the finished format. If there is color or photos that go all the way to the edge, you typically let the graphics go 3 mm beyond the edge. This is because the printing sheet is always larger than the printed matter itself, as it is trimmed afterwards.
If you don't bleed, you risk leaving a thin white line around the edge where you would otherwise like to print all the way to the edge. This is because you can't always get the cut exactly right and therefore some of the paper that should have been cut off is left behind. Therefore, always make sure to bleed when printing to the edge. It's important that you don't forget to activate your bleedsettings when exporting your PDF files to print. Otherwise, there is a chance that the 3 mm bleed will not be included in your finished PDF file.
Read more about how to create print-ready PDF files here.
