Valence Shell Electron Pair Repulsion Theory
Very basic stuff you do in high school chemistry. Has to do with the molecular geometry of believe it or not, molecules. VSEPR Theory centres around the principle of arranging the valence orbits of a central atom in a molecule to minimise the repulsions overall. Thus the orbital arrangements simply depend on the number of (effective) electron pairs.
It's not really important, just a pain becuase almost all 1st year University chemistry subjects will go over it again, and again, and again.