What it means is that these sims' outfit assignments have 'gone bad.' Not the outfits themselves, but the assignments to them. One way to fix that is to somehow get the sims to change their outfits - actives are easy, have them go swimming or take a shower, but generally we can use MC to select a new existing outfit for them to wear, even the one they are already supposed to be wearing, and the problem is solved for that sim going forward.
Alternatively, or in addition, DebugEnabler has a Fix Invisible Sims command that redoes these outfit assignments on all sims in the world it thinks have 'bad' ones all at once. Or are you seeing something else entirely happening? Many of us find the DE method to be at least 80-90% effective. When I went to 'have those beers' and realized just how late it was I just went to bed.
