Why is potassium electron configuration 1s2 2s2 2p6 3s2 3p6 4s1 or Ar 4s1?

Ar is Argon and Ar 4s1 is the short form of the electron configuration 1s2 2s2 2s6 3s2 3s6 4s1. It means add 4s1 to the electron configuration of Argon to get the electron configuration of potassium.