First things first, this isn't English, and for it to sound English it's pronounced "vesper". It stands for "valence shell electron pair repulsion." It's main purpose is to predict the shape of the molecule with its linear geometry and molecular geometry.
The main idea of VSEPR theory is that the valence electron (pairs) surrounding an atom repel each other. They therefore create a shape that minimizes the tension and repulsion, and gives a molecule a shape.
Unequal distribution of the forces that pull against the central atom is polar.
Equal distribution of the forces that pull against the central atom is non-polar.
Ionic bonding consists of taking electrons, and for that they cause weird shapes or no shapes at all.
ELECTRONEGATIVITY AND POLARITY
Each element has its own electronegativity value. The difference in the values is what shows the polarity of the molecule.
Common Atoms and their Electronegativity
Using VSEPR Theory
The molecular geometry (arrangement of atoms to minimize tension) can determine the polarity of a molecule.
Atoms Bonded with Central Atom
Lone Pairs of Electrons
Bent Trigonal Planar
There are more scenarios, but these are common.
You will notice the molecular geometries that are bent are polar, this is because of uneven distribution causing the bent shape.
Carbon Dioxide (CO2)
Non-Polar: Equal Distribution
Polar: Unequal distribution due to an unshared electron pair on Nitrogen, the central atom.
Why should I care?
As previously mentioned, VSEPR theory predicts the shape and angle of molecules. This is practical because it will show if a substance will mix with another substance based on it's lewish structure of or formula. (Like disolves like.) The picture on the left shows oil (non-polar) and water (polar) don't mix because of polarity. In a nutshell, predictions on a molecule's geometry shows how it reacts with others, thus predicting even more scenarios.