A fill card is a card of any size - from index card up to 2'x4' or even larger - than is used to reflect light onto a subject to light up an area that otherwise is a little too dark. Sometimes the fill card is placed behind a contained filled with liquid (like a glass of beer) to make it pop and sparkle! Why does that come to mind now?
I remember an interesting incident in my 2nd year at RIT. I was in line at Grace Watson Dining Hall, and a girl I was meeting for a date showed up wearing a very low-cut outfit. I forget exactly what the circumstances were, but she had to bend over to reach something. And when she did - I remembered instead of wishing for a more direct view, I wished I had a fill card to throw a little light onto the subject. At first I thought that was odd, but then I knew once again I was "hooked" on photography - which is Greek for "writing with light".
Years later I would work for photographers who mostly let me set up a shot for them, then come in and tighten things up. Most of the time that was done for the benefit of the clients (if they were in the studio). One time I remember being able to see from an off-camera angle a fill card was sticking out from behind a prop. I diplomatically and quietly (yes I can be both when the situation calls for it) mentioned it to the photographer who took credit for the save. That only happened rarely when I was working as an assistant.
But more often I see really bad lighting in photography these days. I am always amazed at the photography I see where the photographer doesn't realize that they are writing with light. Where the light is just haphazardly placed without care of what the light does in all parts of the shot. Like one model who is well-lit but casts a shadow on the face of another model who you now can't see very well.
Could it be the reliance on digital imagery and Photoshop, or the lack of clients who go out on shoots with photographers these days? Or the fact that so many photographers who shoot today never came up as an assistant before shooting?
Surely some folks do say that they can take care of it in Photoshop, but why not take care of it on set so you can save time post-production?