The inertial mass of an object is a property of the object linking the acceleration of the object relative to a field in space that I will term the temporal inertial (TI) field. This linkage resists the acceleration of the object relative to the TI field. While inertial mass is a property of the object, it is the TI field, not the object, that asserts the force that resists the acceleration of the object relative to the TI field. This force is termed the inertial reaction force and it is a real, not a fictitious force. The inertial reaction force is a physical force between an object and the TI field. It is not a force derived from a change in a frame of reference. In one instance, the inertial reaction force manifests as the weight of an object at rest on the surface of a gravitational body, such as Earth. It derives from the acceleration of the TI field at the object and toward the center of mass of Earth. This interaction occurs because the TI field is directly subject to gravity and matter objects are not. Accordingly, the TI field is accelerated toward the center of mass of Earth. Absent air resistance, a free falling object is accelerated at the same rate as the TI field toward the center of mass of Earth. It is the acceleration of the TI field toward Earth that determines that all objects, regardless of mass, accelerate toward Earth at the same rate, at the same rate as the TI field! Ever wonder what the acceleration of gravity means? It is the acceleration of the TI field in response to gravity. The roles of inertial mass and the inertial reaction force are examined in five different scenarios to illustrate their interaction.