A horizontal flip swaps left and right, like a reflection in a mirror. A vertical flip swaps top and bottom. Neither is the same as a rotation: rotating by 180 degrees turns the picture upside down but does not reverse text, while a horizontal flip makes text read backwards.
Turning on both flips at once gives the same result as a 180 degree rotation. If that is what you want, the Rotate tool does it directly and handles several images at once.