polarity is a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment. Polar molecules must contain polar bonds due to a difference in electronegativity between the bonded atoms.
When two atoms of different electronegativities are combined together then more electronegative atom attracts the valence electrons of less electronegative atom.
For example, in a NaCl molecule sodium has a partial positive charge and chlorine has a partial negative charge.
As a result, a partial negative charge will develop on the electronegative atom. And a partial positive charge will develop on the less electronegative atom.
