Why CuCl is less ionic than NaCl and what is the meaning of polarisability?

In Cucl , cu+ is having pseudo electronic configuration. So the presence of d electrons which shows poor shielding effect pulls the electron cloud of anion , hence more covalent character.
Polarizabilityis a measure of how easily an electron cloud is distorted by an electric field. Typically the electron cloud will belong to anatomor molecule or ion. The electric field could be caused, for example, by an electrode or a nearby cation or anion.