The simple answer –
It is the control of unwanted heat loss through the building envelope and putting in place a design to stop it with the use of various products such as vapor control layers.
The long answer –
Building air tightness (also called envelope air tightness) can be defined as the resistance to inward or outward air leakage through unintentional leakage points or areas in the building envelope. This air leakage is driven by differential pressures across the building envelope due to the combined effects of stack, external wind and mechanical ventilation systems.
Airtightness is the fundamental building property that impacts infiltration (the uncontrolled inward leakage of outdoor air through cracks, interstices or other unintentional openings of a building, caused by pressure effects of the wind and/or stack effect).