A dipole is formed when two atoms with different electronegativities form a covalent bond. In HCl, the Chlorine atom is more electronegative than the Hydrogen atom, so it attracts electrons towards itself, creating a slight negative charge (δ-) on the Chlorine atom and a slight positive charge (δ+) on the Hydrogen atom.
The slight negative charge on the Chlorine atom of one molecule can interact with the slight positive charge on the Hydrogen atom of another molecule, resulting in an attractive force. This attractive force between the dipoles of HCl molecules is known as dipole-dipole interaction.
