Very few of the experts here use Macros, but Screen.ActiveControl simply refers to the Control, on the Form the Macro is being called from, that currently has Focus when the Macro is called.
In this particular Macro, it it telling the Access Gnomes that if the current Control is not empty, open the Report whose name is the same as the Control's Value.
I have to add that if you plan on doing much work, in Access, you really need to learn VBA rather than rely on Macros! Macros are very rigid and more limited, in what they can do, compared to using VBA code.
Hope this helps!
The problem with making anything foolproof...is that fools are so darn ingenious!
All posts/responses based on Access 2003/2007