Power smoothing is a feature available in the Selfloops Spark app that is generally used to smooth power values coming from bike power sensors.
Smoothing is generally done by by averaging the last n values coming from the bike power sensor using a moving average formula.
Most power meters measure power throughout the pedal stroke and many high-end smart trainers do something similar. Your power will vary as you are pushing down and pulling up the pedal with each revolution.
If you watch your power in real time – without any smoothing – you will see numbers all over the place.
Some smart trainers allows you to set Erg mode smoothing.
In the Selfloops Spark, in the Setting section, it is possible to select a smoothing value to avoid this problem.