"Bag of freebies" refers to methods that improve the training accuracy of YOLOv4 without increasing the cost of inference. These techniques include various forms of data augmentation like photometric distortions (adjusting brightness, contrast, etc.) and geometric distortions (scaling, cropping, flipping, rotating). By increasing the variability of the input images, these augmentations help YOLOv4 generalize better to different types of images, thereby improving its robustness and accuracy without compromising its real-time performance.