DPI
DPI ("dots per inch") is the resolution measurement that determines how sharp a printed card will look. 300 DPI is the professional print standard; 150 DPI is fine for home prototyping but visibly soft on a press; 72 DPI is screen-only and will look fuzzy on cardstock.
Why it matters
You hit DPI when a print shop rejects a file as "low resolution" or you zoom into a proof and the text looks pixelated. The fix is usually exporting at 300 DPI from the start.
Example
A poker card (2.5" × 3.5") at 300 DPI is 750 × 1050 pixels in the exported file. At 72 DPI the same card is only 180 × 252 pixels — far too small to print sharply.