VBA executes whenever the code is triggered from within your project.
This can be from all sorts of object events from simply opening a form to clicking on a command button.
When something doesn't work as expected, how can you determine exactly what is wrong and where?
Well, that's what this article can help you with.
With every Office application, including Access of course, there is an integrated debugger. To open the window of the debugger and to give you access to all the code associated with your project (There is one project with every database), simply press Alt-F11. This will also switch you between associated Debugger and Application windows when you're currently in either.