it's just that their exact meaning can't be expressed by an exactly equivalent English word. That's almost always the case with translations, at least of abstract concepts.
To be serious for a change,
ardua or
arduus is clearly the latin root for the english word "arduous" which makes the meaning pretty clear to me. Wherever it may have come from, "Per Ardua Ad Astra" is a highly suitable motto for the youngest but certainly not the least of our armed forces.