Explain wrapper class and methods in C#

A wrapper class is any class which "wraps" or "encapsulates" the functionality of another class or component. This provides a level of abstraction from the implementation of the underlying class and "hides" the implementation from the outside world.
For example, if you write a music player, you would probably have a Track class which handles where it is on disk, the band name, album, and so forth. But you would also want a playlist: which could be a List<Track> quite easily. But it might make more sense to create a "wrapper" class called Playlist which contains a list of tracks, but exposes the Add and Remove methods, which also has a name and knows how to save / load itself to disk. Because the actual List is hidden from the outside world, they can't change the content without you knowing about it and being aware that the list needs to be saved.