It is a property of a material where the bulk resistivity is charged by mechanical stresses applied to the material. If you apply mechanical stress or force on a bulk material, then its property particularly resistivity will change. That is the piezoresistivity.
Piezoresistivity which is very important and which is used for making micro sensors, either it is a many kinds of micro sensors, MEMS micro sensors like the pressure sensors, gyros or accelerometer. They are based on the piezoresistivity property of silicon materials.
Change of the strain by application of change of resistance is basically the gauge factor.
For higher sensitivity, a high value of gauge factor is desirable. A high gauge factor means a relatively higher change in resistance which can be easily measured with a good degree of accuracy. Semiconductor strain gauges are used where a very high gauge factor and a small envelope are required. They depend for their action upon piezoresistive effect i.e. the change in the value of the resistance due to change in resistivity. Semiconducting materials such as silicon and germanium are used as resistive materials.