"über" as a prefix can mean "too (much)". "Überforderung" is "too big a challenge", "too much of a challenge". If somebody's job is too challenging for them, then too much is demanded of him, i.e. he is overworked. (Or maybe he just isn't the right person for the job and should never have been hired in that capacity, but let's not talk about that, okay? ;) ) Does that make things clearer? All the best, Susanne

It's something like a challenge with no excessive demands. *I'm not sure.