A bleed is the outside edge of a print product that is trimmed off. When creating a print document, we add an extra 0.125" around all edges of a product for the bleed. This way, when the product is trimmed down to size, the printed area goes all the way to the edge of the product.
In the example below, the red area is known as the bleed. It is a border that is trimmed off 0.125" from the outer edge. Anything in this area will be removed when printed and trimmed to size. Designers fill background in this area so that there aren't slivers of white edges on your printed products.
The blue area is a safe margin. It's 0.125" inside of the bleed (0.25" from the outer edge). It's best to keep your artwork out of this area so that nothing is awkwardly close to the edge or accidentally trimmed off.
The white area is your safe area. Put all text, logos and important artwork in this area.