Because ions in an ionic crystal are tightly bound together, it takes little energy to change them from a solid to a liquid state the charged particles are not free to move and conduct an electric current the ions of like charge strongly repel one another electricity easily flows between them

Ions are particles formed when an atom of an element looses or gains electrons to attain a stable configuration.They are either positively charged (cations) or negatively charged (anions). Ionic crystals consists of ions bound together by electrostatic attraction forming a regular arrangement, geometric structure called crystal lattice. In an ionic solid the ions are held by strong attractive forces (tightly bound) and cannot move from their positions in the crystal structure and  therefore, do not conduct electricity.