Since atoms vary in their atomic number and number of valence electrons, so does its "pull" for electrons towards its core. Which is known as Electronegativity. Think of it as how strong of a "magnet" atoms are for electrons.
So during chemical bonding (namely the Covalent Bond where atoms share electrons), the difference in the bonding atoms' Electronegativity will result in one atom pulling the (negatively charged) electron more towards it, disrupting the harmony of the electrical charge balance.
This slight asymmetry or "tilt" in the electrical charge balance will result in the molecule to have slightly negatively charged and slightly positively charged sections in its structure, making it "polar".
I hope that answered your question.