"Your long finger."
Translation:Do chorrag fhada.
When you need to lenite it – you lenite it (and in writing you add h to mark the lenition), I don’t know what other explanation you expect. There are many contexts requiring lenition, won’t list them all in a comment here.
They are explained in the tips and notes to the lessons when they are introduced. You can read the tips and notes in the web browser version of Duolingo (on https://duolingo.com) – unfortunately not in the mobile app – or on the duome.eu website.