Mass Loading is the effect caused by adding an additional mass to a dynamic structure.  The aim of any measurement is to measure the response of a system as if the measurement equipment is not present.  By adding a mass to a dynamic structure you will change the dynamic behaviour of the structure.  It is important to minimise this change to ensure you are getting the highest quality measurement.  It is recommended to keep the ratio of mass of the item under test to the mass of the sensor below 10:1.  However, it is important to remember that the cable and connector add mass to the system as well as the sensor.  If you are measuring a very lightweight object to will need to consider the total mass you are adding to the system.  A simple test can be made to understand the mass loading effects.  If you take a measurement of the system with the sensor attached and then repeat the measurement with an additional mass attached at the measurement point equivalent to the mass of the sensor, connector and cable.  The difference between these two measurements will give you an idea of the effect on the results of adding the mass of just the sensor.