HSV, also known as HSB, is a way of specifying colors. It is similar to RGB, but instead of stating how much red, green, and blue is in the color, it states the hue, saturation, and value of the color.
Hue, Saturation, and Value
Hue
Hue is the color of the color- red, green, blue, yellow, etc.
Saturation
Saturation is how much pigmentation is in the color.
Value
Value, also known as brightness or lightness, is how dark a color is.
Understanding:
Here's a color picker. Moving to the right, you get more saturation, or color. Moving to the left, you lose color and it goes to gray- desaturation. The value would be up and down. And the hue exists on the color bar to the right of the big box.