Bleed is the industry term for any color or image that goes right to the edge of the paper. What actually happens is that the picture or other graphic extends 3mm beyond the edge of the page and that excess image or color is then cut off as a part of the bindery or finishing process to ensure there is no 'white space' or borders where graphics are desired to extend to the the very edge of the page.
Text or images that are not meant to be trimmed off the edge of your final printed piece must be 3mm from the edge of your layout (also known as Safety). Items placed 3mm or closer are in danger of being trimmed off or showing inconsistent margins.
If you would like to learn how to setup bleed in your InDesign document, be sure to look at the FAQ How do I setup Bleed in my InDesign Document? which talks about this in more detail.