Convolution filtering is the process of averaging small sets of pixels across an image.
Convolution filtering is used to change the spatial frequency characteristics of an
image (Jensen 1996).
A convolution kernel is a matrix of numbers that is used to average the value of each
pixel with the values of surrounding pixels in a particular way. The numbers in the
matrix serve to weight this average toward particular pixels. These numbers are often
called coefficients, because they are used as such in the mathematical equations.