Question

what is meant by noble gas core?

Answer 1

Noble gas core is a term used for electronic configuration of noble gases.
Noble gas configuration is a configuration in which the valence shell is full. More specifically, in noble gas configuration, both the s and the p subshells of the valence shell are fully filled and hence cannot accept more electrons.

A noble gas configuration is therefore denoted by ns²np⁶, where n = 2, 3, 4 etc.