Lewis structures, also known as Lewis dot diagrams are diagrams that show the bonding between atoms of a molecule and lone pairs of an electron that may exist in the molecule.
Polar covalent bonds are bonds that occur where a pair of electrons is shared unevenly between two atoms
The Lewis notation as an agreed convection, uses the delta sign to indicate polar covalent bonds when they occur in compounds. It shows that the electrons are shared unevenly.
Consider the example attached below. The Lewis structure for HF is given. the δ+ is used to show the electropositive Hydrogen atom while the δ- is used to show the electronegative Fluorine atom in the bond.