The Advisor is responsible for checking the integrity and quality of your Oracle APEX application.
Oracle APEX applications are interpreted, that's why most of the errors are only showing up when the application is executed and not already
when you are doing the development.
But there are also errors which even do not show up during execution, for example if you reference a
non existing page item with the substitution- (&P999_TEST.) or bind variable syntax (:P999_TEST). Oracle APEX will just ignore it and return NULL instead.
There are also cases where invalid SQL or PL/SQL code is just ignored, see here and here.
That's where the Advisor comes into the game. It's like a compiler/LINT for your Oracle APEX applications, it will check the integrity of your application based on the meta data of your application. It does not only check the above cases, there are currently a total of
22 checks which are performed! You can even add your own QA checks.