An add on is the simplest way to imagine a plug-in.
There are no specific roules about how to create an add-on, all you need to know is
that the file index.php, you created in your plug-in directory, will be included by the system
as long as the plug-in continues to be activated.
So if you, for example, create an index.php that define a function:
function helloWorld() {
echo "Hello World!";
}
Now you are able to use your new helloWorld function in every script of your template.
EasyMagazine cares about inclusion of your function when the plug-in is activated.