An image is composed of pixels and each pixel, can contain one value (one channel) that generates a gray image, or can contain three values (RGB or three channels) that generate a color image. Each channel contains values from 0 to 255 (from a black to a saturated channel, in the case of a one-channel pixel from black to white). Depending on the content of the image, you will get different amounts of each gray value.
A histogram is a simple table that gives you the number of pixels that have a given value in an image (or sometimes, a set of images). The histogram of a gray-level image will, therefore, have 256 entries (or bins). Bin 0 gives you the number of pixels that have the value 0, bin 1 gives you the number of pixels that have the value 1, and so on. Obviously, if you sum all of the entries of a histogram, you should get the total number of...