Dyne level is surface energy expressed in dynes/cm (equivalent to mN/m). It is measured with dyne test inks or pens: apply the ink, and if the film holds as a continuous line for two seconds the surface is at or above that value; if it beads up, it is below.
The practical rule is that substrate surface energy should exceed the coating's surface tension by roughly 8-10 dynes/cm for reliable wetting and adhesion. Solvent-based inks need around 38 dynes/cm on film. Water-based systems, with higher surface tension, generally need 40-44.
Because corona treatment decays over time, the dyne level on the roll you are about to run is the number that matters — not the number on the certificate that shipped with it. Checking it takes seconds and prevents most film adhesion failures.