Covalent bond is formed by sharing of electrons. When two dissimilar atoms having different electronegativities combine to form a covalent bond, the bond pair of electrons will not be shared by both the atoms equally. As a result, the bond pair shifts towards the nucleus of the atom having greater electronegativity. Thus, electron distribution gets distorted and the electron cloud is displaced towards the electronegative atom. The electronegative atom becomes slightly negatively charged while the other atom becomes slightly positively charged. Thus, opposite poles are developed in the molecule and this type of a bond is called a polar covalent bond.
Example of polar covalent bond is HCl molecule. In HCl molecule, Cl is more electronegative than H. As a result, electrons cloud gets displaced towards chlorine and it gets partial negative charge whereas hydrogen gets slightly positive charge.