 All models are based on principles or rules that govern how the model works. The basic ideas behind the Lewis model are that atoms in a molecule are held together by shared pairs of electrons, called bonding pairs, that are located between the two atoms being bonded, and that the remaining valence electrons may be distributed as non-bonding pairs or lone pairs. The VSEPR model takes those principles and adds a few of its own. As the name suggests, it's all about the fact that electrons repel each other since they all have the same charge, they're all negatively charged. So VSEPR says that in a molecule, bonding and non-bonding electrons will repel each other and that the molecule will adopt a shape that allows those electrons to be as far apart as possible. Hence, valence-shell electron pairs, that's the bonding and non-bonding electrons, repel each other.