Private application and library code. This is the code you don't want others importing in their applications or libraries.
You can optionally add a bit of extra structure to your internal packages to separate your shared and non-shared internal code. It's not required, but it's nice to have visual clues showing the intended package use. Your actual application code can go in the /internal/app directory (e.g., /internal/app/myapp) and the code shared by those apps in the /internal/pkg directory (e.g., /internal/pkg/myprivlib).