A pixel is a rectangular area of color and is the smallest area of color that a computer monitor can display. Digital images are made up of thousands, sometimes millions, of pixels arranged in a 2D plane. Pixels are very small and so when the image is displayed on a computer monitor we do not normally see the individual pixels. In the image below, a digital picture has been magnified and a grid placed over the top. Each grid square contains a single pixel of the original image.
Each pixel is a block of solid color. The color of a pixel can be represented in a number of ways. From our perspective these are all equivalent and can be reduced to the canonical Red-Green-Blue mixture representation. Now consider that each pixel can be referenced via its x and y coordinates. In digital images the point of reference is usually taken as the upper left, so that the x coordinate increases left-to-right and the y coordinate increases top-to-bottom. Using this idea, the image below shows a 4x4 square of "pixels" with their x and y coordinates written in the form (y,x).
Now we know that the color of a pixel is determined by its red, green, and blue components and that we can assign coordinates to pixels. The core concept behind evolving images (using the methodology I did), is that we can generate pictures by making the red, green, and blue color components of each pixel, a function of its coordinates. That is, the color of each pixel is a function of its position. Aesthetic evolution is nothing other than the "breeding" of the underlying functions of the "most pretty" images for thousands of generations, in a way analogous to the selective breeding of livestock.