The value of a color is simply the lightness or darkness (in black-and-white) of that color. If you were to take a black-and-white photograph of that color, it would register as one of the values on the value scale. Red is always a dark value (7-9) and yellow is always a light value (2-4). Yellow ochre is more of a middle value (4-6) 

If you painted a panel with all your favorite colors and then took a black-and-white photograph of them, you would see the values of your favorite colors.