Kernel
A small grid of weights slid over an image to blur it, sharpen it or find its edges.
See it in FX engine. Under Adjustments, raise Sharpness: a small kernel is doing the work.
In plain words
A kernel is a tiny grid of numbers, often three by three. It is placed over each pixel in turn, covering that pixel and its neighbours. Each neighbour is multiplied by the number above it, and the results are added up. That sum becomes the new pixel. Change the numbers, and the same method blurs, sharpens or traces edges.