When halftone dots print larger on the press than what they originally were on the plate or film, resulting in a loss of detail and lower contrast in the image.
This results in some loss of detail. Some degree of dot gain is an unnavoidable part of the printing process and there are settings in a number of desktop publishing tools to allow for this.
It is predictable to a point and can be compensated for when film and plates are produced Dot gain often occurs in long press runs, due to plates and/or pressure settings wearing or changing through out the run.
Adobe Photoshop, in particular, has settings to allow for dot gain under it’s color settings preferences.