Compare with shielding .Electrons in penetrating orbitals can reach the nucleus. The n and quantum numbers determine how well an orbital penetrates. Lower n and lower values mean better penetration. A low n value means the orbital is small. A low value means the orbital has fewer nuclear nodes (planes that pass through the nucleus where the probability of locating the electron is zero).
In order of decreasing penetration, the subshells are s > p > d > f. A 1s orbital penetrates better than a 2s orbital.