How to add a description to the Goals used by the Goals module?
Goals used by the Goals module are implemented using fieldable entities. So you can customize goals by adding your own fields, such as:


descriptions.
images.
links to external resources.
etc.


Head over to its video tutorial, and have a look at what is shown starting from around 4:00 where you can see the typical tabs for Goals, such as:


Manage Fields.
Manage display.


For more details, refer to its Community Documentation also.

Disclosure: I'm the maintainer of this module.