One binary digit is a bit. Group eight of them and you get a byte — 256 possible values.

Hexadecimal (base 16) exists purely as a convenience: one hex digit covers exactly 4 bits, so two hex digits describe a whole byte cleanly — far easier for a human to read than a string of eight zeros and ones.

You already used one today, probably without noticing: a colour code, a MAC address, a cryptographic key — all hexadecimal shorthand for binary underneath.