Blurring refers to averaging the pixel values within a neighborhood. This is also called a low pass filter. A low pass filter is a filter that allows low frequencies and blocks higher frequencies. Now, the next question that comes to our mind is—What does "frequency" mean in an image? Well, in this context, frequency refers to the rate of change of pixel values. So we can say that the sharp edges would be high frequency content because the pixel values change rapidly in that region. Going by that logic, plain areas would be low frequency content. Going by this definition, a low pass filter would try to smoothen the edges.
A simple way to build a low pass filter is by uniformly averaging the values in the neighborhood of a pixel. We can choose the size of the kernel depending on how much we want to smoothen the image, and it will correspondingly have different effects. If you choose a bigger size, then you will be averaging over a larger area. This tends to increase the smoothening...
