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Value is simply the darkness (low key) or lightness (high key) of a color. This is most readily discernible in the gray scale, where black fades to white. In web color, you only have several "web safe" gray values to chose from - From black (00) to the next lightest color (33) and up again to 66, 99, CC and then to FF. This is part of the hexadecimal system of web color.
In illustration mediums outside of computer generation, values can consist of hundreds of steps. This is not limited by the standard of safe web colors.