What does "Experience as a superior" mean?
As a Finn, and without further context, I read that as leadership experience. I would translate "superior" as "esimies". Esimies is Finnish for superior (of someone at work, in the sense that they are responsible and have power over other people; not in the sense that they are superior as human beings or scientists or anything like that). Have you lead a research group or supervised PhD students? Have you been the principal investigator of some project?

I don't think there is any precise definition available.

If the recruitment system is also available in Finnish or Swedish, you might also look at the equivalent term there and check what it means.