Dot gain is generally understood as the actual growth of a dot both physically and by what can be measured optically by using a microscope. In simpler terms it is the difference in dot size from what was output on a film or digitally to your plate to what finally appears on the printed substrate.
Densitometers actually measure both the physical and optical change in the dot to calculate its theoretical final size. There are many variables that affect dot gain from the actual ink system, plate durometer, thickness, ink film thickness, sticky back, plate to substrate impression, anilox to plate impression and ink metering system. With new digital to plate technology, dot gain can be compensated for greatly thanks to the ability of being able to produce dots of 1 thou or less.
In terms of print quality, dot gain is generally undesirable particularly in high light areas and when trying to produce vignettes. While it is mostly commented on in relation to the size or growth of an individual screened dot the same effect occurs with text and line images, which can cause fill in and general blurring of the final finished graphic. With a flexible plate generally the thicker the plate the more dot gain is generally seen and the same can be said for plates that are produced using traditional negatives.
New digital software, thinner plates, improved sticky back and laser output devices are doing a great deal in reducing this issue but it is still dependent on the press and its operator to achieve a minimal impression set to achieve the least possible dot gain.