[Winga]

It means the chances of an e- in an orbital to penetrate the core electrons to the nucleus?

[Mitch]

What do you mean by "core electrons"?

[Winga]

Inner electrons!

[Carbon13]

Are you talking about penetration and shielding of electrons and how it can affect the effective nuclear charge?

Sheilding electrons occupy penetrating orbitals.  The electrons closer to the nucleus act as a shield for electrons further away from the nucleus, shielding them from the nuclear attraction.  This can be explained by considering orbital radial distance function.