Flipping makes a mirror copy of a picture. A horizontal flip swaps left and right, as if you were looking at the photo in a mirror. A vertical flip turns it upside down along the horizontal axis (which is different from rotating it 180°: a vertical flip also mirrors it). Choosing both gives the same result as a 180° rotation.
The most common use is fixing front-camera selfies, which many phones save mirrored so text on a T-shirt or a sign reads backwards. Designers also flip product shots and portraits so a subject faces into the page, and teachers flip images for symmetry exercises. Select up to 30 files and they are all flipped the same way in one run, with a live preview of each before you save.
Flipping only moves pixels, so it never blurs the image. The file is saved in its original format by default, and JPG and WebP are re-compressed at 92% quality. Saving removes EXIF data such as GPS location. Animated GIFs are flipped as their first frame.