An accelerometer measures the acceleration of an object, or in other words the rate of change of an objects velocity. It also uses earth`s gravity on a static object and measures its movement or vibrations. The most typical sensors have 3 axes, oriented on the x, y and z planes. One of the most common usages of accelerometer would be the orientation sensor in smart phones.
As we already know, accelerometers are analog sensors that provide a different amount of voltage for each axes. To connect it to the bela we need 5 wires, 3 for the axes, one for the power and the last one for the Ground.