In net ionic equations, not all reactant species participate in the reaction.
A complete ionic equation can be used to express a double replacement reaction. In complete ionic equation, dissolved ionic compounds are shown as their free ions.
Ions that appear on both sides of the complete ionic equation are the ions that did not directly take part in the reaction. These ions are called spectator ions.
Separating those spectator ions from the equation leaves us with the net ionic equation which expresses only the particles (ions) that took part in the reaction.