Regular packages are packages that contain an __init__.py file.
The __init__.py file is used to mark directories on disk as Python package directories.
In the simplest case, __init__.py can just be an empty file.
But it can also execute initialization code for the package or set the __all__ variable.
The __all__ variable is used to define what symbols are exported when from <package> import * is used on the package.
See this and this for more information.
Here is an example of a regular package:
package/
├── __init__.py
├── module1.py
└── module2.py