A digital image consists of pixels collected in matrices. A matrix can be thought of as a table, where the columns and rows give the width and height of the image. Each cell of this table represents one pixel of the image.
Often digital images are stored in such a way that there are tables for one basic color each (red, green, blue), which are added together for display. Each of these tables thus represents a monochrome image, which, when superimposed, produce the colored image. Each individual pixel of a monochrome table is represented by a numerical value. In normal computer graphics, there are 2^8 (265) monochrome gradations from black to white. Three tables result in 256256256(16777216) possible color values.
If you understand an image as a combination of three matrices, you have the possibility to make changes to these matrices based on mathematical algorithms. Many filters in Photoshop are based precisely on the fact that images are subject to these mathematical laws.