Set up the artboard right the first time
Bleed is the part of the design that gets trimmed off. Get the numbers wrong and you ship a white edge. Enter your trim size and this does the arithmetic.
How this is calculated
Three concentric rectangles. The artboard adds bleed to every edge, so it istrim + (bleed × 2) in each dimension. The safe zone pulls in from every edge, so it istrim − (safe × 2). Set your document to the artboard size, place the trim line at the original dimensions, and keep anything you can't afford to lose inside the safe zone.
Switching units converts the values you've entered (1 in = 25.4 mm), so you can design in whichever your printer's spec sheet uses. When in doubt, send the file to yourprint designer or read what a job typically runs on theprint design cost guide before committing to a run.