Answer
It is used to represent the core electrons.
Work Step by Step
Writing the electronic configurations of elements with a high atomic number can be a tedious process. Therefore in order to ease the task, we represent the core electrons of the element with the symbol of previous noble gas.
Example- In order to write the electronic configuration of Francium, instead of writing the entire configuration, we can simply write $[Rn]7s^{1}$.
Here Radon is representing the previous noble gas and is representing the core electrons, whereas $7s^{1}$ indicates the valence electrons.
