The electron configuration of an atom is expressed in a form of notation which shows how the electrons are distributed among its various energy levels and orbitals.
The format for writing it is this: 1s2
The number 1 refers to the energy level (in this case, energy level 1). The "s" stands for subshell that the electrons are occupying. The superscript number 2, (raised) tells us the number of electrons in that subshell.