What does it mean when it's said it's a "16 bit key" or "64/128 bit key"

Question

I need some detailed explanation on the length of the key? What it means when someone says it's a 16 bit key or 128 bit key etc. The Wiki definition is " In cryptography, key size, key length, or key space refer to the number of bits in a key used by a cryptographic algorithm "

Referring to the statement from Wiki, the next question is: How to determine number of bits in a key?

I'm getting confused as there is no example specified, could anyone advise with simple examples?

Answer 1

A key is a sequence of bits. The length is the number of bits in the key. A simple example of a 16-bit key would be 1010 1010 1010 1010 (0x5555 or decimal 21,845). Any sixteen-bit number. A 128-bit key would be any 128-bit number.