The sampling depth or bit depth is a measure of the dynamic range of a signal. It applies equally to audio signals, to the color differences of photos, images and graphics, and to the brightness differences of video signals.
The bit depth itself says something about the level differences between the values of two samples. Small level differences indicate that the samples are fine and the sampling of the analog signal has many level differences. This means that the analog signal was sampled at a higher sampling depth.
With a sampling depth of 4 bits, 16 step values can be assigned to the analog levels. With 8 bits there are already 256, with 16 bits 65,536 and with 24 bits 16,777,216 step values. In relation to a physical quantity, an analog signal quantized with 8 bits would have 256 level differences assigned to it. For a color signal, there would be 256 color differences. Modern displays can show videos with color depths of 21 bits, which corresponds to a color representation in Direct Color, but also in True Color with 24 bits, with which over 16 million color differences can be represented. The highest color resolution is offered by Deep Color, which works with a color depth of up to 48 bits.