Compounds are formed by the bonding between different atoms. The bonds usually formed are either ionic or covalent in nature.
No bond is purely covalent or purely ionic in nature. There exist some amount of ionic or covalent nature in them. Such bonds are said to be polar bonds.
Covalent bonds are formed by the sharing of electrons between similar atoms, where the electrons are equally shared among them. Example: etc.
In these, the electrons will remain at an equal distance from both identical atoms. This results in the formation of non-polar bonds.
In the case of some compounds, there is an electronegativity difference between the two atoms. This difference results in the shared electron to be pulled more closer to the most electronegative atom. The bond thus formed between the two compound has a polar bond.
Example: In the molecule , fluorine is more electronegative than hydrogen. This results in the shared electron to be pulled more closely to the fluorine atom in the molecule. Thus, the covalent bond is said to be polar.
Thus, the electronegativity differences between the atoms results in the polarization of bonds.
This polarization of bond result in the molecule possessing a dipole moment, which in turn contributes to its magnetic nature.
Thus, a polarized bond is a covalent bond that posses some amount of ionic character in them due to the electronegativity difference.