Answer :
Dipole moments occur when there is a separation of charge. Dipole moments occur due to atoms electronegativity, where one atom has the ability to attract electrons towards it giving it a negative charge and the one deficient in electrons acquire a positive charge called as the bond moment.
But if the bond moments are equal and opposite in direction , they cancel each other and thus there is no net dipole moment in the molecule.
For example: In carbon dioxide , both the -C=O bonds are polar but as the molecule is linear and the the magnetic moments are equal and oppposite, they cancel each other and the molecule is non polar.
