TouchWiz is an interface developed by Samsung dedicated for phones created by this company. The first version debuted with the Samsung Galaxy S in 2010, and from the beginning there appeared controversial opinions about it. A lot of people complained about lagging phones, and Samsung had worked on it in every new version of TouchWiz.
Today, mainly by the photos which leaked from beta of Android 7.0 Nougat we noticed change of the name TouchWiz into Samsung Experience. Great changes in the interface are not detected yet, and Samsung probably decided to change the name of it mainly by the fact that people associate the name TouchWiz with lagging phones and unstable interface.