Glossary
DPI
Also called: dots per inch, ppi
DPI (dots per inch) measures how much detail a file carries per unit of printed size. Print artwork is normally prepared at 300 DPI at final size; screen graphics at 72 DPI will look noticeably soft when printed.
DPI is only meaningful together with physical size. A 1000-pixel-wide image is 300 DPI across three inches and 100 DPI across ten, so a file that is perfect on a mug can be unusable on a poster.
Increasing DPI in an editor without re-creating the artwork adds pixels but no detail. If a file is too small, the fix is to rebuild it larger, not to resample it.