Each instance of a package can have paramaters associated with it. These are like preferences, and they can be set by the administrator for each application to change the behavior of your application.

To add parameters for your package, go to the Automatic Package Manager (/acs-admin/apm)

Click on your package

Under the Manage section, click on Parameters

It's fairly self-explanatory at this point. Create the parameters you want, and then access them in your code using the parameter::get procedure.