Bit depth or better known as colour depth is referring to the number of bits presented in one pixel of colour. Bits represent all the 0’s and 1’s that specify a certain colour. Different shades of colours can be displayed with higher bit rates.
You can calculate how many colours there is in a bit depth by multiplying 2 by itself for the amount of bits
ex. (24 = 2x2x2x2 = 16) which in this case means that 4-bit has a total of 16 colours.
As technology progressed the evolution of colour depth grew too. From monochrome displays which were 1-bit (black and white) up to our modern computers which standardize to the 32-bit (4,294,967,296 of different colour combinations). The 48-bit is also being introduced but video cards that support this colour depth are not as common yet.
Examples of colour depths:
1-bit (monochrome) – 2 colours
4-bit (Standard VGA) – 16 colours
8-bit colour – 256 colours
16-bit colour (high colour) – 65,536 colours
24-bit colour (true colour) – 16,777,216 colours
32-bit, 36-bit, or 48-bit colour (deep colour) – >4,294,967,296 colours