What is Sour Crop in Chickens and How to Treat It

I remember walking out to check on the girls one hot summer day a few years back I do regular health checks on my flock, so I immediately noticed Nugget, my black Silkie, hanging back by the fence looking off One look and I knew she wasn’t feeling right I picked her up and felt her crop, and when I bent over to grab something off the ground, a smelly liquid shot out of her mouth Gross! That was my first experience with sour crop in chickens