Question

What is the meaning of 1s2 electron configuration (tell what 1 means, the s means and the superscript of 2 means)?

Answer 1

The electrons around an atom occupy orbitals called shells in a fixed order.  The s orbital is the first and smaller and can hold 2 electrons.
In this case, there are 2 electrons in this atom as 2 electrons are in the s orbital.