Bleed is extra print area which gets cut off during trimming.

See below... black crop marks shows where the cut occurs, green line is the finish size.

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What will happen with no bleed?

Cutting down prints that have colour to the edge will never be perfect to the mm..
 

So if there's no extra print outside the trim line, the cutter will usually result in a slither of white edge on top, bottom and or sides of the print.

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DOWNLOAD BLEED EXAMPLE PDF