A digital image is a grid, with a set of numerical values at each location in the grid. Each location in the grid is known as a pixel, the smallest addressable element, and the values at that location represent the color of that pixel. The upper left corner of the grid is location (0,0) and we move from left-to-right and top-to-bottom.
In an RGB color image there are 3 values in the set. Each value represents the intensity of Red, Green, and Blue light respectively; with 0 representing no light and 255 representing maximum intensity.
A channel is the value for a specific color intensity. We can isolate a specific channel by setting the intensity of the other channels to zero (0). In the following figure we show an original image and then isolate each of the channels in the image.
In a grayscale image there is only one channel. In grayscale, 0 indicates black and 255 indicates white.
Note that there are other color spaces used for color images, but RGB (Red, Green, Blue) or (Blue, Green, Red) are the most commonly used ones. An application typically expects an image in a particular color space so it may be necessary to convert between color spaces.