Bleed is the amount of image area that goes beyond the trim down size. There are a number of reasons why printers need bleeds, some of which are covered below.
The red border represents the trim size. Notice how the image of the house goes beyond the trim, that is correct bleed. Standard bleed for offset printing is 1/8th" (.125")
At the press
As press sheets come off the press, sheets vary slightly in their landing position. To reduce this variation, stacks are jogged. However, it's always best practice to include bleeds because it only takes a fraction of an inch to leave a white edge when cutting down the job.
Panels may need to be adjusted by our prepress and or bindery staff. With bleeds, this is usually not a problem. Without bleeds, jobs with images going out to the edges may look inconsistent from one piece to the next or from edge to edge.
First box - good registration
Second box - bleed with imperfect registration
Third box - no bleed, imperfect registration
Note the differences between the examples with bleed vs without bleed.