Thin white lines along one or more edges after the job is trimmed.
Filecheck fixes this automatically at upload
Cutting is mechanical and mechanical things have tolerance. A guillotine trimming a stack of sheets will wander by a fraction of a millimetre, and every sheet in that stack moves slightly differently.
If your background stops exactly at the trim line, any drift toward the inside of the sheet exposes the unprinted paper underneath. The result is a thin white sliver along one edge, usually not on every copy, which is what makes it so annoying to argue about afterwards.
Bleed is the fix: extend the artwork past the trim line, normally by 3mm, so that wherever the blade lands it is still cutting through your design.
Bleed protects against cutting short. The safe zone protects against cutting long. Keep text and anything else that must not be clipped at least 3mm to 5mm inside the trim line.
Together they mean the blade has somewhere to be wrong in both directions without the result looking wrong.
Most often the artwork was designed at exactly the finished size. That is the intuitive thing to do, and it is correct for a screen. It is also the single most common reason print files have to go back to the customer.
Files exported from design tools aimed at social media, or from office software, essentially never carry bleed, because those formats have no concept of trimming.
Missing bleed is detected at upload and, where the artwork allows, added automatically by extending the edge content outward. Where the design cannot be safely extended, because something meaningful sits right at the edge, the upload is flagged rather than guessed at. Either way, it is resolved before the job enters production.
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