"Value" refers to how light or dark an object or shape is.
The scale often depends on the artist but I work on a scale of 0 to 10. 0 being white and 10 being black or vice versa depending on the piece I am working on. As you can see in the value scale finder below. Black is 1 and White is 10. I always think of white paper as 0 value.
In order to create depth, your value has to be correct. Sometimes you can put any colors you want and if your value is right. The Painting/Drawing will look great. As my favorite artist, Joseph says "Tone is always the king"
Here is an image of a value scale. You can buy this is on amazon or any art supply store.
Now, let us discuss how to do value studies. You can either pick different subjects online from sites like Flickr, Google, mapcrunch, etc., or highly recommend doing these studies outside in the real world. It helps you to train your eyes.
Here is a demo:
Below is the same image both in color and in grey scale. I numbered the values based on the light and darkest tones. You can also do this study to help you to understand and learn tones in colors.
Now it's your turn, Download the image and number it. you get better as you do it
If you have any questions, comment below. Happy painting!!