What is Relative permittivity?

In a medium, the Coulomb force between two charges is divided by a number $\varepsilon_r \ge 1$, the relative permittivity (or dielectric constant) of the medium:

It is large for liquids of polar molecules, which orient around the charges and screen them: $78.4$ for water at $25\,{}^{\circ}\mathrm{C}$, $1.89$ for hexane at $20\,{}^{\circ}\mathrm{C}$.