The Motion Sensor is looking for a warm object moving across its field of view. If it detects movement and heat it will trigger an alarm.
Some common triggers include things such as heating vents, space heaters, pets that are over 85 pounds or able to climb up onto a piece of furniture near the Motion Sensor.
If there is anything in the environment that the Motion Sensor can see that you believe may be causing the alarms, move the Motion Sensor to a new location to avoid these possible triggers.