Normalized Power (NP or Weighted Average Power) is an estimate of the power that you could have maintained for the same physiological "cost" if your power had been perfectly constant.
This metric smooths out the variability in the power of your ride caused by different factors (e.g. terrain, wind, grade, etc.) to provide a better indicator of your overall effort and allows you to compare your effort between different rides.
Normalized power is an important metric which we use to calculate the Training Stress Score.