The blur effect is another common Photoshop filter. It is a simple example of a convolution. A convolution is an image processing operator that calculates a weighted sum of neighboring pixels.
Here, the variable d gives the distance from one pixel to the next. It is configured for a 512×512 texture.
In this case, the center pixel contributes the most to the output. The pixels in the cardinal directions contribute half that, and the corner pixels contribute a quarter. The result is a simulation of an out-of-focus camera.