Value adds the most to works of art, but is the most difficult concept for students and people to learn. Think about it. We're surrounded by shapes and objects, and you're used to interacting with those objects and spaces. Then you go and ask someone to look at it longer, to literally break up what they're seeing into separate pieces.
Value is composed of an infinite number of shades depending on how close you look. Let's start with just three parts to get you started though. There's no point in losing our mind over a simple concept like value.
Highlights - the lightest parts; where light is on top of subjects. If you have a baby elephant under banana tree, and the sun is beating down on him the top where the sun is beating on him is the highlights.
Shadows - shadows are easy to spot. They're the parts usually farthest away from the highlights. They're under the elephant's arms, and on his belly. They are the parts you can't see into. They're the parts where the elephant disappears into darkness. Non-poetically, the black parts. (I won't even get into why that's not a good enough explanation, but let's accept it for now until you're a master.)
Mid-tones - these are ALL the colors inbetween highlights and shadows. Think of these as everything except white and black. Simple explanation, but it'll work for you until you fall in love with all of the infinite shades you can't wrap your brain around (neither can I, which is why they're so exciting!). There are mid-tones in shadows and highlights if you isolate them too.
A great way to explore value; get some watercolors!
STEP 1 - Choose a face. Sketch with pencil. My student and I were looking at anime and manga proportions. The hair is the real place we explore value. Try not to use colors straight out of the tube, almost no color comes straight out of the tube except for dog toys and legos. Mix everything with its complementary
STEP 2 - Apply the first layer of lightest hue. We used a shade of lightened blue and added layers of darker bluish indigos. Build up, deepening the shadows. Leave areas exposed for highlights. That's how you develop value with watercolors. Build up.
STEP 3 - Do the same with a flesh colored orangey-pink hue. Add up to deepen value. For an anime or manga look leave her very light, for darker skin hues like the illustration below cover the whole thing in a wash and then add up. Pay attention to shiny highlight areas, and darker areas beneath eyes. Your overall washes end up being your mid-tones.