All bonds are not created equal. Some bonds perfectly share their electrons, and others barely share them at all. A dipole moment is the mathematical expression that describes this disbalance.
The sum of all dipole moments is equal to the net dipole. If all dipoles perfectly cancel out, the molecule with have no net dipole.
Example: Do the following molecules contain dipole moments? Which contain net dipoles?
Hint: You will have to draw the correct Lewis Structure first .6m