Smart filters were introduced in Photoshop CS4. “Convert for Smart Filters” allows you to change a filter you applied to a layer earlier. For example, if you’ve converted a layer for smart filters and added blur, you can go back later and remove some blur without losing the original fidelity of the image. They increase your file size tremendously, but if you’re going to save the final version as a jpeg for the web, it’s not a bad idea.