This is the rare adverb: in a way that cannot be found out or calculated exactly. You might see it in careful writing: two results differ indeterminably, or a wait stretches indeterminably (you cannot tell when it will end). It is formal and uncommon. Students almost never need to say it; they only need to recognise it if it appears. Though actually, even native speakers often reach for “in a way we cannot measure” or just “endlessly” instead. Same core idea as indeterminable, but it modifies a verb or an adjective rather than a noun.