A wetness indicator is a feature in many disposable diapers that helps to alert the parent as to when the diaper is exposed to liquid and therefore might need changing.  Rather than wrangling your little ones in an hourly rodeo to check if they need changing, let the diapers do the work for you. Usually, the feature is as simple as a color-changing strip, or a design that fades or gets more pronounced as it detects moisture.