Many yoghurts are sold in pots with thin (foil or plastic) film lids. Due to the hazards of transit, yoghurt (and in many cases, fruity goodness) often ends up forming a delicious layer on the film.
Cunnilidgus is the practice of using one's tongue to recover this flavoursome goodness.
Yoghurt envier: Hey can I have some of your yoghurt?
Yoghurt owner: No, but you can have the lid.
Yoghurt envier: Great, is there a spoon?
Yoghurt owner: What is this, the Ritz? Time for cunnilidgus, my friend!