This can be used to rotate the image for an interesting effect, or more commonly to correct the image if it is at a slight angle.
If your photo is too bright or dark, you can adjust the brightness in order to get a well-balanced image.
You can take photos within the app. This allows you to do everything from capturing the image to editing all within one app, instead of having to import an image taken with another camera app.
Adjusting the contrast affects the difference between light and dark areas of the image. You can fine tune it to meet your needs, such as increasing the contrast to make your subject stand out more.
Saturation refers to the intensity of colors. It is measured on a scale ranging from 0% (black and white) to 100% (pure colors) at a constant level of brightness. Images with high levels of saturation have vivid colors, while unsaturated images appear greyish.