An overlay is a part of the graphical user interface that is rendered in the viewport, i.e. the part of the GoSimulate window where the models are displayed.
In order to create the overlays, GoSimulate uses a special directory on the hard drive. This directory is defined by the operating system, and you normally need not care about it.
However, one user reported a problem where the overlays were not being rendered correctly, thus in effect making the application impossible to use. If you see a screen such as the topmost of the below images when starting GoSimulate, the source of the problem is probably the same.
In the case with this particular user, the problem was that the operating system defined a cache directory to which the user had no writing rights, which in effect made it impossible for the application to create the overlays.
If you have this problem, you might try to explicitly change the cache directory. For instructions about how to to this, refer to this section.