How does a covalent bond become polar?
Covalent bond becomes polar on the account of electronegativity difference between two atoms involved in bond formation.
Lets us consider first a non-polar covalent bond.
See both atoms have the same electronegativity. Therefore, they share electrons of the covalent bond equally between each other.
On the other hand, if we consider any polar covalent bond like
Chlorine being the more electronegative atom has the tendency to draw the shared pair of electrons towards itself and hence generate polarity.
All and all, unequal sharing of electrons actually makes a covalent bond polar.
Hope it helps!!