Transmissivity is the rate at which water is transmitted through a unit width of an aquifer under a unit hydraulic gradient. It is expressed as the product of the average hydraulic conductivity and thickness of the saturated portion of an aquifer.
Transmissivity is a measure of the ease with which groundwater flows in the subsurface.
Transmissivity is used to calculate the yield of a borehole, determine the safe yield of an aquifer system and predict groundwater movement.
Source: http://www.ust.smoothstone.com/groundwater_intro