"Bleed" is a printing term used to refer to the extra printed area around the outside edge of your artwork we need to trim off to make it look like you art goes all the way to the edges. In reality, printed material that has art extending to the edges need to be created a little bit larger than the final size.
Imagine a tablecloth that was the exact same size as your table, you need a little extra to go over the edge a printer would call that extra part a bleed.
In printing, we need an extra 1/8” "bleed" of any artwork or background design that will extend to the edge to go beyond the final trim size on each side of your printed piece. To be clear, the "bleed" must be a continuation of all artwork/graphics that go to the trim edge of your card.
After your project is printed, it is put into big stacks and taken to a large trimmer to be cut to its final size. When the stacks of paper are placed in the trimmer, they are jogged and adjusted to align top to bottom, clamped in place and then a large blade drops down through the paper cutting away the bleed area. The result is very accurate, but there is some amount of fluctuation, the extra "bleed" ensure that this fluctuation is hidden.
***Don't Get Cut Off*
Remember to design so that all vital information such as phone numbers and addresses are 1/8" inside the final trim line of your print job. This is known as the critical print area.