what does training instance mean?

I am new to machine learning. I just stumble across the term 'training instances' in a paper about using CNN for image segmentation. In that paper, a total 1100 images were used for modeling. The authors chose sub-regions from the images for training, and they presented a classification performance curve over 500K training instances. I am confused about they get such a large number of training instances from only 1100 images. Does one training instance mean one training sample or something else related to the training size?