Do you know what a constructor does? I think you might need to go back to a basic tutorial on them, as they'll be able to explain it much better than I can. The Java tutorials have some information
here.
Once you know what a constructor is for...sometimes it needs some information to be able to do it. Providing an object might be one way of providing that information. In the Box example, you needed to supply the dimensions to initialise the Box. Providing another Box with those dimensions is just one way of doing it.