An impacted crop is when food gets stuck inside a bird's crop, hardens, and blocks digestion.
This can happen for a few reasons:
1. It can happen if a mother bird doesn't shell seed properly and eats them whole, then regurgitates it for their young. Young birds cannot digest whole seeds, leading to crop impaction.
2. It can happen if a bird overeats and can't digest all the food its ingested in time before the food begins to harden.
3. It can happen if a bird eats something indigestible that blocks up the crop, resulting in what's in the crop hardening.
Treatment varies bird to bird. With larger birds, such as chickens, you can sometimes help them drink water and massage the impaction away. Smaller birds, like parakeets or cockatiels, will need to go to a vet immediately. I've heard that some people feed their hens apple cider vinegar to get the birds to vomit the impaction up, but I've also heard that this is a bad idea that can go terribly wrong if the crop has already gone sour, or if there's an infection.
This is what birb had when she was a baby.