Bit depth, also known as color depth, can refer to two different instances. It either gives information about the number of bits used to make out the color of one single pixel itself, or its color component. This information is used in bitmapped image and video files alike.
The value of Bit Depth determines the amount of tones an image or video is able to display. The higher this number, the more tones can be represented and vice versa. Greysacle images usually have a Bit Depth from 2 to 8, but are also able to be represented by higher Bit Depth values. Colored images usually have a bit depth ranging from 8 to 24 or even higher.
When talking about a 24 bit image, the bits are often grouped to represent the three basic colors of the RGB spectrum. This means they contain 8 bits for red, 8 bits for green and 8 bits for blue. These colors, in combination, can be used to represent many different color tones (2²⁴ values in total).