The mathematical word for it is a convolution. Basically you take each pixel in the image and replace it with a wider blurrier pixel.
Imagine you're playing the game Battleship and you have pegs in all your ships. Now replace each peg with 9 pegs - one on the same spot, and 8 surrounding that spot. Now your ships will be bigger, and if you had any 1 peg gaps between ships, those gaps would have disappeared. This makes the image "blurrier" by making sharp edges go away.
It's a little bit more complicated than that... your outer pegs would be "less bright" than your center peg, and you would have "ocean pegs" that bleed onto the ships, so you would end up with a blurrier image of your boats.