Measurements from the real world often contain noise. Loosely speaking, noise is just the part of the signal you didn’t want. Maybe it comes from electrical noise: the random variations you see when calling
analogReadon a sensor that should be stable. Noise also arises from real effects on the sensor. Vibration from the engine adds noise if you’re measuring the acceleration of a go-kart, for example. Filtering is a method to remove some of the unwanted signal to leave a smoother result.
Filtering Noisy Arduino Measurements – [Link]