The Blur filter makes your image look out of focus by randomly displacing pixels. You can alter the amount by which the image is blurred (the distance that pixels are displaced) by adjusting repeat value (between 1 and 100).
The Random Seed is how GIMP calculates which pixels to displace — you can either set a numeric value which will cause all repeats (if a repeat value is set) to use the same seed, or you can use the current time, which will mean that each repeat will use a different seed.