‘Crystalline solids are anisotropic in nature’. What does this statement mean?

All crystalline solids are not anisotropic. Those crystalline solids which are anisotropic have their atoms arranged and spaced in a different manner in three different planes (X, Y and Z). Therefore, the physical properties of crystalline solids such as electrical resistance or refractive index show different values when measured along different directions in the same crystals.

Example NaCl, Quartz, Ice, HCl, Iron, etc.