which often stands in for:
you didn't use a flash to take this picture of a person, and therefore it sucks
To be fair, sometimes it does mean "needs fill" but even then it's wrong.
There are several things going on here. At least, but not limited to:
- pros use lights, therefore to get a professional look you must use lights.
- contemporary lighting has wildly open shadows, so a contemporary look has just enough lighting ratio to produce modeling.
- the use of diffused lighting flatters the skin in a couple of ways.
Experiment: Take any old snapshot of a person. Use a curves adjustment to lift the skin tones and reduce contrast in the skin tones (grab the middle of the curve and lift straight up a little). Then apply any sort of skin smoothing effect you like, for example duplicate the layer, blur it slightly, and make the blurred layer quite translucent. Poof. You're 90% of the way to a contemporary portrait look, because the contemporary portrait look isn't about the lighting at all. All, amazingly, without the use of fill light/reflector of any kind.
When we see a picture of someone just standing around outside, there are a bunch of "tells" that this is not a professional photograph, that this was not shot with a softbox or beauty dish. There's an assumption in play that everyone want to make pictures that look like professional portraits: magazine covers, wedding portraiture, school pictures, corporate headshots, and so on. These things are always subject to contemporary standards, and currently that standard has several properties. The hidden assumption is that this family of "looks" is correct, and other looks are incorrect.
Fill flash, or the use of a reflector, produces some but not all of those properties. Which is where the idiotic parroting of "needs fill" comes from.
Professional lighting is about having lights, not about where you put them. There's a name for pretty much anyplace you put them, after all. Some placements flatter some subjects but not others. Some placements flatter no subject at all. Any placement at all produces a modest degree of "professional look". Because pros use lights, your pictures don't look professional without lights, and everyone wants to make pictures that look like what the pros are crapping out this week, right?
How much of the contemporary look is caused by simply possessing a great pile of lights? When you've got a beauty dish left, a couple softboxes right, something on the background, and a hair light, and you're not out of monolights, what are you to do? One thing you do is you throw more lights into the mix. If all you have is 1000 hammers, everything looks like 1000 nails. Now you've got light all over the place and, lo, the shadows are mostly obliterated. Pores and other skin texture vanishes under the assault of a dozen diffused lights. Now your picture looks a bit like a contemporary "Good Housekeeping" cover. Congratulations. I guess.
Obviously professional portraiture isn't all silly, in fact it's quite complicated and can be done very very well. The point is there's a specific look that the "needs fill" remark is referring to, that has nothing much to do with how good the picture is. The look applies to terrible portraits as well as excellent ones, and to all the ones in between.
Can you imagine people looking at Rembrandt's paintings, clucking their tongues, and saying "needs fill"? I can, and then I smile because I am imagining Rembrandt punching these dolts in the face repeatedly.