Great question! The measure feature width function is used to determine the width of a trench or line feature in addition to its height. A line is a feature above the surface plane, and trench is a feature below the surface plane. The feature selected for measurement is determined by the location of your S1 and S2 points.
You can see where the feature has been identified in the line profile after calculating the step height result. It will be identified as the W1 point, with a horizontal bar indicating the width measured.
If you have multiple trench or line features, you can also enable the average feature width option. ProfilmOnline detects any other features of the selected type, then gives an average width along with the number of features identified.
The image I've shared below is a good example to test out the feature. Feel free to use it experiment and better acquaint yourself with how this function works.
Feature Width Example