A blower door test measures the air-tightness of homes and buildings and can help detect air leakage sites in the building envelope.
The blower door is mounted on an exterior door and will pressurize or depressurize a home in order to measure the air changes in a home. Air changes per hour (or air change rate) is a measure of the air volume added to or removed from the home (or space) divided by the volume of the home (or space). If the air in the home is uniform, air changes per hour is a measure of how many times the air within a defined space is replaced.
Ultimately this test will help determine how energy efficient your home is and thus how many air exchanges you have.