Dot gain is the increase of the halftone dot size that occurs during the pre-press and printing process.
It creates what appears to the human eye as a darkening on the image.
This is also referred to as tone value increase. It is the difference between the requested value and the final result. It can be anticipated and controlled by using a densitometer to determine the percentages of change needed during film output.
Most printers will not need to worry about this phenomenon due to the innate lack of high detail in most screen printed garments. If you have a high detail image requiring precise dot management it is best to get a densitometer and record the percentage changes in your Rip software for a more accurate output.