Very, very basically it is a schnapps from the Essen area of Germany: A traditional and strong spirit. If memory serves, it takes its name from a local island. I am going out on a limb here: A literal translation of the world "Friesengeist " might be something like "the essence of a military fever". "Geist" means 'ghost' 'spirit' 'soul' 'imagination' and so on. "Friesen" (plural of Friesel) is ( sorry) blisters associated with what is known as military fever. I'd rather think of it the name as a charmingly parochial way of describing the effects of this herbal beverage. Schnapps is, virtually by definitiion, a potent substance. My relatives never bothered lighting it before consuming. From their perspective, why burn off the alcohol? But, a little bit of ceremony adds to the festivities!
