printf – problem to understand code

__( 'Remove Track' ) is a translatable string. __(), _e(), _x(), … all of them are gettext functions used by WordPress for language localization. It is missing the second parameter (optional), which is the textdomain, usually it should be:

__( 'Remove Track', 'your-plugin-text-domain' )

In that way, the string can be translated by your plugin without any conflict if the same string exists also in another plugin or in core.

You can get more details in I18n for WordPress Developers.