The WINSxS folder (also known as Windows Side by Side) holds the code for installing the roles and features of Windows and the installation files for other applications that you install on your client. This folder allows you to install additional components without asking you for the installation media. This is a good thing as we move closer to the day when DVD drives will be found only in museums.
The big thing here if the number of GB that this folder consumes. Microsoft suggest not deleting anything from this folder as it could hinder your ability to add additional functionality.
The following link to the blog site for Microsoft Enterprise Platforms Support: Windows Server Core Team will give you some options on how to possibly reduce the size of the WinSxS folder safely.
As always, backup and test before putting their procedure into production.