Image effects are Standard Assets that provide a quick, simple way to change the look of your game. It’s easiest to think of them like an image-editing application that adds filters to your images (such as Photoshop or even Instagram).
Image effects in Unity work by providing an extra post-processing pass per frame, manipulating the pixel content to alter the image. For this reason, image effects can be resource-intensive, and this should be taken into consideration when adding them to your build. The more image effects you add, and the more heavily you process your images, the lower your framerate is likely to be as a result.
A number of image effects ship with the Unity Editor; many more are available on the Asset Store and elsewhere online. To learn more about the image effects available in the Unity Editor, see the Image effect reference guide.