ValueStack is the storage area where the application data is stored by Struts2 for processing the client requests. The data is stored in ActionContext objects that use ThreadLocal to have values specific to the particular request thread.
Object-Graph Navigation Language (OGNL) is a powerful Expression Language that is used to manipulate data stored on the ValueStack. As you can see in architecture diagram, both interceptors and result pages can access data stored on ValueStack using OGNL.