Bleed is the area of a design extending beyond the trim line (usually 3–5 mm) that prevents white slivers along the edges after cutting. Prepress checking (preflight) is the technical inspection of a file before it goes to the printer.
Bleed exists because cutting machines trim paper with a certain tolerance. If a background or photo ends exactly on the trim line, the slightest shift of the blade leaves a white sliver along the edge. That's why elements reaching the edge are designed to extend past the format, usually 3–5 mm depending on the printer and printing technique. For the same reason, important content (text, logos) stays within a safe margin from the trim line.
Preflight is the file check before printing: colours in CMYK (plus any Pantone), bitmaps at 300 DPI, fonts embedded or converted to outlines, correct bleed and margins, overprint settings and the right colour profile. Printers publish their own specifications, and PDF/X is the standard for file exchange.
Why it matters for business: in print there's no fix after publishing. A mistake caught in the file costs fifteen minutes of work; the same mistake discovered after printing costs the whole print run and the deadline.